FIX: Show user filter hints when typing @ in search (#13799)

Will show the last 6 seen users as filtering suggestions when typing @ in quick search. (Previously the user suggestion required a character after the @.)

This also adds a default limit of 6 to the user search query, previously the backend was returning 20 results but a maximum of 6 results was being shown anyway.

app/models/user_search.rb

@@ -12,6 +12,7 @@ class UserSearch
     @topic_allowed_users = opts[:topic_allowed_users]
     @searching_user = opts[:searching_user]
     @include_staged_users = opts[:include_staged_users] || false
+    @last_seen_users = opts[:last_seen_users] || false
     @limit = opts[:limit] || 20
     @groups = opts[:groups]
 
@@ -162,6 +163,15 @@ class UserSearch
         .each { |id| users << id }
     end
 
+    # 5. last seen users (for search auto-suggestions)
+    if @last_seen_users
+      scoped_users
+        .order('last_seen_at DESC NULLS LAST')
+        .limit(@limit - users.size)
+        .pluck(:id)
+        .each { |id| users << id }
+    end
+
     users.to_a
   end