Use new secondary thread tags everywhere.

Decided to just update the images and move them around on the relevant views.

Fixes #277.
Delete impossible-to-fulfill TODO.

SA has no known way of filtering a forum by secondary thread tag.
Fetch state-restored page of posts if nothing is cached.

Can happen after an update when Core Data's automatic migration fails.
Delete private API showing user's wallpaper in basement.

If it goes public, we can add it back.
Grab screenshot of Awful 2.0.
Use device date formats (Issue #283)
Show prompt if cookie expires soon (Issue #287)
Prevent crash when no filter icon is selected