Add Ignore Slash Command

This followed the same template as !65 (merged), the one thing we can't do yet in Slash commands and have an autocompletable list of Mentions. So for the /ignore command we'll have to add one User at a time.

This also doesn't make it so that ignored Users are persisted across recording sessions. In fact, there's little change to any of that, the Pawa API is simply keeping a map of SessionID -> List, which we may use later as the source of truth.