(core) support python3 in grist-core, and running engine via docker and/or gvisor

Summary:
 * Moves essential plugins to grist-core, so that basic imports (e.g. csv) work.
 * Adds support for a `GRIST_SANDBOX_FLAVOR` flag that can systematically override how the data engine is run.
   - `GRIST_SANDBOX_FLAVOR=pynbox` is "classic" nacl-based sandbox.
   - `GRIST_SANDBOX_FLAVOR=docker` runs engines in individual docker containers. It requires an image specified in `sandbox/docker` (alternative images can be named with `GRIST_SANDBOX` flag - need to contain python and engine requirements). It is a simple reference implementation for sandboxing.
   - `GRIST_SANDBOX_FLAVOR=unsandboxed` runs whatever local version of python is specified by a `GRIST_SANDBOX` flag directly, with no sandboxing. Engine requirements must be installed, so an absolute path to a python executable in a virtualenv is easiest to manage.
   - `GRIST_SANDBOX_FLAVOR=gvisor` runs the data engine via gvisor's runsc. Experimental, with implementation not included in grist-core. Since gvisor runs on Linux only, this flavor supports wrapping the sandboxes in a single shared docker container.
 * Tweaks some recent express query parameter code to work in grist-core, which has a slightly different version of express (smoke test doesn't catch this since in Jenkins core is built within a workspace that has node_modules, and wires get crossed - in a dev environment the problem on master can be seen by doing `buildtools/build_core.sh /tmp/any_path_outside_grist`).

The new sandbox options do not have tests yet, nor does this they change the behavior of grist servers today. They are there to clean up and consolidate a collection of patches I've been using that were getting cumbersome, and make it easier to run experiments.

I haven't looked closely at imports beyond core.

Test Plan: tested manually against regular grist and grist-core, including imports

Reviewers: alexmojaki, dsagal

Reviewed By: alexmojaki

Differential Revision: https://phab.getgrist.com/D2942

plugins/core/manifest.yml

@@ -0,0 +1,24 @@
+name: core
+version: 0.0.0
+description: Grist core features
+components:
+  safePython: sandbox/main.py
+contributions:
+  fileParsers:
+    - fileExtensions: ["csv"]
+      parseFile:
+        component: safePython
+        name: csv_parser
+    - fileExtensions: ["xls", "xlsx", "tsv", "txt", "xlsm"]
+      parseFile:
+        component: safePython
+        name: xls_parser
+    - fileExtensions: ["json"]
+      parseFile:
+        component: safePython
+        name: json_parser
+
+scripts:
+  build:
+  # Note that ${XUNIT:+xxx} inserts "xxx" when XUNIT is set, and nothing otherwise.
+  test: $GRIST_PYTHON -m runtests discover -v -s /sandbox ${XUNIT:+--xunit}