(core) move data engine code to core

Summary:
this moves sandbox/grist to core, and adds a requirements.txt
file for reconstructing the content of sandbox/thirdparty.

Test Plan:
existing tests pass.
Tested core functionality manually.  Tested docker build manually.

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2563

package.json

@@ -5,6 +5,7 @@
   "main": "index.js",
   "scripts": {
     "start": "tsc --build -w --preserveWatchOutput & webpack --config buildtools/webpack.config.js --mode development --watch --hide-modules & NODE_PATH=_build:_build/stubs nodemon -w _build/app/server -w _build/app/common _build/stubs/app/server/server.js & wait",
+    "install:python": "buildtools/prepare_python.sh",
     "build:prod": "tsc --build && webpack --config buildtools/webpack.config.js --mode production",
     "start:prod": "node _build/stubs/app/server/server"
   },