A couple of tweaks to the appearance of the new history feature.
Expire cached revisions of deleted pages. Tweak the appearance of "history" page a bit.
Fix config/routes.rb to make revisions cacheable. Cache revisions. Modify the history page so that it links to the current page (and diff).
From Jason Blevins: Create a "History" page for each wiki page. Link to it, and to the "Diff" page from "Recently Revised". Also, correct a bug in listing/deleting links to uploaded video and audio files.
