FEATURE: Allow showing hashtag autocomplete results without term (#19219)

This commit allows us to type # in the UI and present autocomplete
results immediately with the following logic for the topic composer,
and reversed for the chat composer:

* Categories the user can access and has not muted sorted by `topic_count`
* Tags the user can access and has not muted sorted by `topic_count`
* Chat channels the user is a member of sorted by `messages_count`

So in effect, we allow searching for hashtags without a search term.
To do this we add a new `search_without_term` to each data source so
each one can define how it wants to handle this logic.

lib/discourse_tagging.rb

@@ -271,6 +271,7 @@ module DiscourseTagging
   #   exclude_synonyms: exclude synonyms from results
   #   order_search_results: result should be ordered for name search results
   #   order_popularity: order result by topic_count
+  #   excluded_tag_names: an array of tag names not to include in the results
   def self.filter_allowed_tags(guardian, opts = {})
     selected_tag_ids = opts[:selected_tags] ? Tag.where_name(opts[:selected_tags]).pluck(:id) : []
     category = opts[:category]
@@ -427,6 +428,10 @@ module DiscourseTagging
       builder.where("id NOT IN (SELECT target_tag_id FROM tags WHERE target_tag_id IS NOT NULL)")
     end
 
+    if opts[:excluded_tag_names]&.any?
+      builder.where("name NOT IN (?)", opts[:excluded_tag_names])
+    end
+
     if opts[:limit]
       if required_tag_ids && term.blank?
         # override limit so all required tags are shown by default