Add explicit link back to editor on the revisions screen

[jenmylo]

Getting back to the post editor on the revisions screen is done by clicking the post title. While this is a standard in the admin in the editor, it can be confusing to some. Adding a "Return to post editor" link in smaller body text with a left arrow before it just under the "Compare Revisions" headline would make it more explicit how to get back to where you came from.

This ticket created based on a request from Matt.

[DrewAPicture]

Related/Duplicate of #24682

[ocean90]

Duplicate of #24682.