Provide 'bin/cache', for managing caches

Summary:
See https://github.com/facebook/phabricator/issues/323. We have a very old cache management script which doesn't purge all the modern caches (and does purge some caches which are no longer in use). Update it so it purges all the modern caches (remarkup, general, changeset), no longer purges outdated caches, and is easier to use.

Also delete a lot of "this script has moved" scripts from the last few rounds of similar cleanup, I believe all of these have been in master for at least several months, which should be enough time for users to get used to the new stuff.

Test Plan: Ran bin/cache with various arguments. Verified caches were purged.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D5978