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/components/site_settings/db_provider_spec.rb

@@ -14,7 +14,7 @@ describe SiteSettings::DbProvider do
   end
 
   # integration test, requires db access
-  it "act correctly" do
+  it "acts correctly" do
     setting = Struct.new(:name, :value, :data_type)
 
     SiteSetting.destroy_all