Allow diffing between the layout tempstore and forward revision

Problem/Motivation

The drupal core experience for layout editing leaves some to be desired. While core warns users that changes to the layout have been made, there's no way to figure out exactly what the changes are!

Steps to reproduce

1. Change something in a layout override, but don't save it.
2. Come back 6 months later and try to figure out what changed.
3. Curse the spirits.

Proposed resolution

1. Provide a mode that lets content editors intuitively compare the forward revision to whatever is stored in the temp store for the current entity.
2. This should render in a modal of sorts via AJAX (?).

Remaining tasks

Prototype

User interface changes

Replaces the "Changes have been made..." status message with one that's a bit more helpful.

API changes

TBD

Data model changes

TBD