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.

plugins/chat/lib/chat_channel_hashtag_data_source.rb

@@ -44,4 +44,27 @@ class Chat::ChatChannelHashtagDataSource
   def self.search_sort(search_results, _)
     search_results.sort_by { |result| result.text.downcase }
   end
+
+  def self.search_without_term(guardian, limit)
+    if SiteSetting.enable_experimental_hashtag_autocomplete
+      allowed_channel_ids_sql =
+        Chat::ChatChannelFetcher.generate_allowed_channel_ids_sql(
+          guardian,
+          exclude_dm_channels: true,
+        )
+      ChatChannel
+        .joins(
+          "INNER JOIN user_chat_channel_memberships
+            ON user_chat_channel_memberships.chat_channel_id = chat_channels.id
+            AND user_chat_channel_memberships.user_id = #{guardian.user.id}
+            AND user_chat_channel_memberships.following = true",
+        )
+        .where("chat_channels.id IN (#{allowed_channel_ids_sql})")
+        .order(messages_count: :desc)
+        .limit(limit)
+        .map { |channel| channel_to_hashtag_item(guardian, channel) }
+    else
+      []
+    end
+  end
 end