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
