This commit removes any logic in the app and in specs around
enable_experimental_hashtag_autocomplete and deletes some
old category hashtag code that is no longer necessary.
It also adds a `slug_ref` category instance method, which
will generate a reference like `parent:child` for a category,
with an optional depth, which hashtags use. Also refactors
PostRevisor which was using CategoryHashtagDataSource directly
which is a no-no.
Deletes the old hashtag markdown rule as well.