FIX: Better handling for toggling must_approve_users

If you turn it on now, default all users to approved since they were
previously. Also support approving a user that doesn't have a reviewable
record (it will be created first.)

This also includes a refactor to move class method calls to
`DiscourseEvent` into an initializer. Otherwise the load order of
classes makes a difference in the test environment and some settings
might be triggered and others not, randomly.

spec/models/category_spec.rb

@@ -434,7 +434,7 @@ describe Category do
     end
 
     it 'triggers a extensibility event' do
-      event = DiscourseEvent.track_events { @category.destroy }.first
+      event = DiscourseEvent.track(:category_destroyed) { @category.destroy }
 
       expect(event[:event_name]).to eq(:category_destroyed)
       expect(event[:params].first).to eq(@category)