(core) add machinery for self-managed flavor of Grist

Summary:
Currently, we have two ways that we deliver Grist. One is grist-core,
which has simple defaults and is relatively easy for third parties to
deploy. The second is our internal build for our SaaS, which is the
opposite. For self-managed Grist, a planned paid on-premise version
of Grist, I adopt the following approach:

 * Use the `grist-core` build mechanism, extending it to accept an
   overlay of extra code if present.
 * Extra code is supplied in a self-contained `ext` directory, with
   an `ext/app` directory that is of same structure as core `app`
   and `stubs/app`.
 * The `ext` directory also contains information about extra
   node dependencies needed beyond that of `grist-core`.
 * The `ext` directory is contained within our monorepo rather than
   `grist-core` since it may contain material not under the Apache
   license.

Docker builds are achieved in our monorepo by using the `--build-context`
functionality to add in `ext` during the regular `grist-core` build:

```
docker buildx build --load -t gristlabs/grist-ee --build-context=ext=../ext .
```

Incremental builds in our monorepo are achieved with the `build_core.sh` helper,
like:

```
buildtools/build_core.sh /tmp/self-managed
cd /tmp/self-managed
yarn start
```

The initial `ext` directory contains material for snapshotting to S3.
If you build the docker image as above, and have S3 access, you can
do something like:

```
docker run -p 8484:8484 --env GRIST_SESSION_SECRET=a-secret \
  --env GRIST_DOCS_S3_BUCKET=grist-docs-test \
  --env GRIST_DOCS_S3_PREFIX=self-managed \
  -v $HOME/.aws:/root/.aws -it gristlabs/grist-ee
```

This will start a version of Grist that is like `grist-core` but with
S3 snapshots enabled. To release this code to `grist-core`, it would
just need to move from `ext/app` to `app` within core.

I tried a lot of ways of organizing self-managed Grist, and this was
what made me happiest. There are a lot of trade-offs, but here is what
I was looking for:

 * Only OSS-code in grist-core. Adding mixed-license material there
   feels unfair to people already working with the repo. That said,
   a possible future is to move away from our private monorepo to
   a public mixed-licence repo, which could have the same relationship
   with grist-core as the monorepo has.
 * Minimal differences between self-managed builds and one of our
   existing builds, ideally hewing as close to grist-core as possible
   for ease of documentation, debugging, and maintenance.
 * Ideally, docker builds without copying files around (the new
   `--build-context` functionality made that possible).
 * Compatibility with monorepo build.

Expressing dependencies of the extra code in `ext` proved tricky to
do in a clean way. Yarn/npm fought me every step of the way - everything
related to optional dependencies was unsatisfactory in some respect.
Yarn2 is flexible but smells like it might be overreach. In the end,
organizing to install non-core dependencies one directory up from the
main build was a good simple trick that saved my bacon.

This diff gets us to the point of building `grist-ee` images conveniently,
but there isn't a public repo people can go look at to see its source. This
could be generated by taking `grist-core`, adding the `ext` directory
to it, and pushing to a distinct repository. I'm not in a hurry to do that,
since a PR to that repo would be hard to sync with our monorepo and
`grist-core`. Also, we don't have any licensing text ready for the `ext`
directory. So leaving that for future work.

Test Plan: manual

Reviewers: georgegevoian, alexmojaki

Reviewed By: georgegevoian, alexmojaki

Differential Revision: https://phab.getgrist.com/D3415

package.json

@@ -6,15 +6,15 @@
   "homepage": "https://github.com/gristlabs/grist-core",
   "repository": "git://github.com/gristlabs/grist-core.git",
   "scripts": {
-    "start": "tsc --build -w --preserveWatchOutput & catw app/client/*.css app/client/*/*.css -o static/bundle.css -v & webpack --config buildtools/webpack.config.js --mode development --watch --hide-modules & NODE_PATH=_build:_build/stubs nodemon --delay 1 -w _build/app/server -w _build/app/common _build/stubs/app/server/server.js & wait",
+    "start": "sandbox/watch.sh",
     "install:python": "buildtools/prepare_python.sh",
     "install:python2": "buildtools/prepare_python2.sh",
     "install:python3": "buildtools/prepare_python3.sh",
-    "build:prod": "tsc --build && webpack --config buildtools/webpack.config.js --mode production && webpack --config buildtools/webpack.check.js --mode production && cat app/client/*.css app/client/*/*.css > static/bundle.css",
-    "start:prod": "NODE_PATH=_build:_build/stubs node _build/stubs/app/server/server.js",
-    "test": "GRIST_SESSION_COOKIE=grist_test_cookie GRIST_TEST_LOGIN=1 TEST_SUPPORT_API_KEY=api_key_for_support TEST_CLEAN_DATABASE=true NODE_PATH=_build:_build/stubs mocha _build/test/nbrowser/*.js _build/test/server/**/*.js _build/test/gen-server/**/*.js",
-    "test:server": "GRIST_SESSION_COOKIE=grist_test_cookie NODE_PATH=_build:_build/stubs mocha _build/test/server/**/*.js _build/test/gen-server/**/*.js",
-    "test:smoke": "NODE_PATH=_build:_build/stubs mocha _build/test/nbrowser/Smoke.js",
+    "build:prod": "buildtools/build.sh",
+    "start:prod": "sandbox/run.sh",
+    "test": "GRIST_SESSION_COOKIE=grist_test_cookie GRIST_TEST_LOGIN=1 TEST_SUPPORT_API_KEY=api_key_for_support TEST_CLEAN_DATABASE=true NODE_PATH=_build:_build/stubs:_build/ext mocha _build/test/nbrowser/*.js _build/test/server/**/*.js _build/test/gen-server/**/*.js",
+    "test:server": "GRIST_SESSION_COOKIE=grist_test_cookie NODE_PATH=_build:_build/stubs:_build/ext mocha _build/test/server/**/*.js _build/test/gen-server/**/*.js",
+    "test:smoke": "NODE_PATH=_build:_build/stubs:_build/ext mocha _build/test/nbrowser/Smoke.js",
     "test:docker": "./test/test_under_docker.sh"
   },
   "keywords": [