Revision tab for all entities

Problem/Motivation

The revision tab is a node thing. There's no general revision UI to show revisions of any entity, nor provides core any kind of helper.
As a result, all non-node entities completely miss such a tab.

Proposed resolution

Diff tries to provide general entity support.
As such, it is the obvious candidate to catch up with the situation and offer a revision tab for all revisionnable entities.
If each entity tries to take care of this on its own, things get easily much more inconsistent and hard to integrate with.

Related: #2424257: Automatic settings integration to all entity types

Remaining tasks

User interface changes

API changes