DEV: Remove enable_experimental_hashtag_autocomplete logic (#22820)

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.

spec/lib/email/sender_spec.rb

@@ -566,7 +566,6 @@ RSpec.describe Email::Sender do
     end
 
     it "changes the hashtags to the slug with a # symbol beforehand rather than the full name of the resource" do
-      SiteSetting.enable_experimental_hashtag_autocomplete = true
       category = Fabricate(:category, slug: "dev")
       reply.update!(raw: reply.raw + "\n wow this is #dev")
       reply.rebake!