Revisions: Fix misplaced buttons in comparison UI.

Align the Previous, Next, and Restore This Revision buttons consistently across viewports on the revisions comparison screen.

Props audrasjb, mokshasharmila13, peterwilsoncc, presskopp, shailu25, wildworks.
Fixes #65062.

git-svn-id: https://develop.svn.wordpress.org/trunk@62273 602fd350-edb4-49c9-b593-d223f7449a82