Glitch::KeywordMute's name is inferred as glitch_keyword_mutes, and in
templates this turns into e.g. settings/glitch/keyword_mutes.  Going
along with this convention means a lot of file movement, though, and for
a UI that's as temporary and awkward as this one I think it's less
effort to slap a bunch of as: options everywhere.

We'll do the Right Thing when we build out the API and frontend UI.

Also add a destroy-all action, which can be useful if you're flushing an
old list entirely to start a new one.

This should eventually be accessible via the API and the web frontend,
but I find it easier to set up an editing interface using Rails
templates and the like.  We can always take it out if it turns out we
don't need it.