Rejigger purge routes

[jfhbrook]

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.