DEV: Remove usages of enable_personal_messages (#18437)

cf. e62e93f83a

This PR also makes it so `bot` (negative ID) and `system` users are always allowed
to send PMs, since the old conditional was just based on `enable_personal_messages`

spec/integration/watched_words_spec.rb

@@ -117,6 +117,7 @@ RSpec.describe WatchedWord do
     end
 
     it "doesn't need approval in a private message" do
+      Group.refresh_automatic_groups!
       manager = NewPostManager.new(
         tl2_user,
         raw: "Want some #{require_approval_word.word} for cheap?",