Remove the bookmarks filter if anon user #398

Attempting to filter by bookmarks when not logged in currently fails hard (see #398)

This patch removes the "Only Bookmarks" link from the sidebar, and ignores `?bookmarks=1` if the user is anonymous.

I'm not convinced removing the link makes for a good UX. Perhaps disabling it in some way is a better option. Let me know what you guys prefer and I'll modify accordingly.