(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

buildtools/prepare_python.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+
+if [ ! -e venv ]; then
+  virtualenv -ppython2.7 venv
+fi
+
+. venv/bin/activate
+pip install --no-deps -r sandbox/requirements.txt