Rejigger purge routes

Right now, wzrd.in has two purge route codepaths. At some point, Max added an "admin" interface which uses GETs to implement purges. Later, without paying much attention, I added unguarded PURGE routes a la varnish.

Right now, both codepaths exist, though the PURGE routes will be disabled Very Soon. I personally think that we should go with guarded PURGE routes, but that seems like a conversation I should have with Max before doing.

Start by comparing the two purge route codepaths described in the issue: the admin GET interface and the unguarded PURGE routes. Resolve with Max whether guarded PURGE routes are preferred, then define the surviving route behavior and disable the other codepath.

Written by the indexing model from the issue text.