DEV: Convert min_trust_to_create_topic to groups (#24740)

This change converts the min_trust_to_create_topic site setting to
create_topic_allowed_groups.

See: https://meta.discourse.org/t/283408

- Hides the old setting
- Adds the new site setting
- Add a deprecation warning
- Updates to use the new setting
- Adds a migration to fill in the new setting if the old setting was
changed
- Adds an entry to the site_setting.keywords section
- Updates tests to account for the new change
- After a couple of months, we will remove the min_trust_to_create_topicsetting entirely.

Internal ref: /t/117248

spec/support/helpers.rb

@@ -39,7 +39,11 @@ module Helpers
 
   def create_topic(args = {})
     args[:title] ||= "This is my title #{Helpers.next_seq}"
-    user = args.delete(:user) || Fabricate(:user)
+    user = args.delete(:user)
+    if !user
+      user = Fabricate(:user)
+      Group.refresh_automatic_groups!
+    end
     guardian = Guardian.new(user)
     args[:category] = args[:category].id if args[:category].is_a?(Category)
     TopicCreator.create(user, guardian, args)