Revisions: Updates to the new Revisions UI.

Various Updates including:
 * i18n fixes
 * Added tracking of what revision ID was restored
 * async fetching of diffs so that slider works sooner even with many revisions

See #23497 props adamsilverstein, ethitter


git-svn-id: https://core.svn.wordpress.org/trunk@23639 1a063a9b-81f0-0310-95a4-ce76da25c4cd