Add generic base classes for content entity revisions

Problem/Motivation

Every content entity that is made revisionable needs to implement a lot of code to support revisions properly. It would be nice to have base classes for the most common revision tasks, so we don't have to duplicate code for each content entity type.

Proposed resolution

Add base classes for:

• Entity revision overview
• View an entity revision
• Revert an entity revision
• Delete an entity revision

Remaining tasks

User interface changes

API changes

Data model changes