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.

plugins/chat/lib/chat/channel_archive_service.rb

@@ -301,9 +301,7 @@ module Chat
     end
 
     def channel_hashtag_or_name
-      if chat_channel.slug.present? && SiteSetting.enable_experimental_hashtag_autocomplete
-        return "##{chat_channel.slug}::channel"
-      end
+      return "##{chat_channel.slug}::channel" if chat_channel.slug.present?
       chat_channel_title
     end
   end

plugins/chat/lib/chat/channel_hashtag_data_source.rb

@@ -3,7 +3,7 @@
 module Chat
   class ChannelHashtagDataSource
     def self.enabled?
-      SiteSetting.enable_experimental_hashtag_autocomplete && SiteSetting.enable_public_channels
+      SiteSetting.enable_public_channels
     end
 
     def self.icon