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/jobs/enqueue_digest_emails_spec.rb

@@ -15,12 +15,13 @@ describe Jobs::EnqueueDigestEmails do
     end
 
     context 'unapproved users' do
-      let!(:unapproved_user) { Fabricate(:active_user, approved: false, last_emailed_at: 8.days.ago, last_seen_at: 10.days.ago) }
 
       before do
         SiteSetting.must_approve_users = true
       end
 
+      let!(:unapproved_user) { Fabricate(:active_user, approved: false, last_emailed_at: 8.days.ago, last_seen_at: 10.days.ago) }
+
       it 'should enqueue the right digest emails' do
         expect(Jobs::EnqueueDigestEmails.new.target_user_ids.include?(unapproved_user.id)).to eq(false)