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
