Summary:
* Extends `/api/docs/docId1/compare/docId2` endpoint with a `detail=1` option to include details of what changed in the document content.
* Adds an `/api/docs/docId/compare?left=HASH&right=HASH` endpoint for comparing two versions of a single document. This is needed to implement the extension to `/api/docs/docId1/compare/docId2`.
* Adds a `HashUtil` class to allow hash aliases like `HEAD` and `HEAD~`.
Everything is a bit crude:
* Changes are expressed as ActionSummary objects, which aren't fully fleshed out.
* Extra data about formula columns is inserted in an inflexible way.
This is extracted and cleaned up from https://phab.getgrist.com/D2600.
Test Plan: added tests
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2614
