Remove references to "slug"

(There are a couple other kinds of slugs in the system, for example, user names, but those are ok, and I haven't touched the ERBs either.)

Many routines just seem to be unused. Not really sure this is the right direction, but wanted to put it out here as a proposal.
`AudiosController#show` is called for routes like `/audio/openvault:2`. We should add a basic test for `AudiosController#show` in `spec/controllers/audios_controller_spec.rb` to cover it. Doesn't need to be too complex... can simply check for returned 200 status code,