Summary:
Render simple differences between documents.
* Show cell changes.
* Show cell conflicts.
* Show row additions/deletions.
Doesn't support any schema changes, and is untested in the presence of schema changes. Any widgets that access row data without using `cells` fields won't receive correct data.
Not addressed:
* Rendering conflicts in mixed row addition/updating/deleting.
* Column additions/deletions, option changes, etc.
* Document level changes.
* Table and column renames (though anticipated in ActionSummary structure).
* Page-level changes.
* Drawing attention to changes (marking changed pages+views, suppressing
unchanged rows, etc).
* Rendering differences in views other than GridView.
* Adding UI for initiating a comparison.
* Editing while comparing.
Replaces {D2600}
Test Plan: added tests
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2618
e5b67fee7e