Work Merge Undo

[mheiman]

Closes #6774

This PR implements a work merge undo interface in parallel with the author merge undo.

Technical

This is largely a copy of the author merge undo interface, though some changes were required to utils.js in the MergeUI in order to pass the metadata needed by the recentchanges interface.

It also relies on a change to infogami/plugins/api/code.py that allows it to accept and pass the save_many data parameter. That will be a separate Infogami PR.

Finally, this PR tweaks the position and appearance of the Undo All button to make it consistent with the other buttons that appear in the header (e.g. Edit) because it was annoying as it was.

Testing

Do a work merge, find it in recent changes, and undo it.

Screenshot

Stakeholders

@mheiman @cdrini