"Diff" view for reviewing editgroups

[bnewbold]

To make reviewing some editgroups faster, would be nice to have a view that displays in "patch" format.

Implementation notes:

• use TOML for the diff. Confirm that python TOML library sorts keys consistently
• for updates, ensure that diff is against current version of entity, not just trusting the "previous revision" pointer. there is a bug in some cases with updating-an-edit which would hide changes
• display entity creation as a link to TOML (?)
• display deletions and redirects similar to how they are already in editgroup review
• endpoint could be /editgroup/<ident>/diff
• have anchors in page with entity name, so can link directly to, eg, /editgroup/<ident>/diff#release_<ident> (or #release_edit_<id>?). then, link to this from entity history list

[bnewbold]

This shipped!