[WIP] Add Entity Audit diff to support revision comparison

Added a `compare action` to the history view, provided by the Entity Audit implementation.

This action will generate a view like the `view revision` action, but with two columns showing the differences between two revisions. (Marked row heading).

Not really sure if I should submit these changes as a pull request, so please advise
I think `currentRevision` values should be tested here as it can take `false` value