Summary:
This adds query parameters useful for tailoring the Grist experience, with an eye to embedding.
Setting `style=light` removes side and top bars, as a first pass at a focused view of a single document page (this would benefit from refining).
Setting `embed=true` has no significant effect just yet other than it restricts document access to viewer at most (this can be overridden by specifying `/m/default`).
Test Plan: added tests
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2585
Summary: This moves enough server material into core to run a home server. The data engine is not yet incorporated (though in manual testing it works when ported).
Test Plan: existing tests pass
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2552
