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/requests/admin/users_controller_spec.rb

@@ -56,12 +56,11 @@ RSpec.describe Admin::UsersController do
   end
 
   describe '#approve' do
+    let(:evil_trout) { Fabricate(:evil_trout) }
     before do
       SiteSetting.must_approve_users = true
     end
 
-    let(:evil_trout) { Fabricate(:evil_trout) }
-
     it "raises an error when the user doesn't have permission" do
       sign_in(user)
       put "/admin/users/#{evil_trout.id}/approve.json"
@@ -70,6 +69,15 @@ RSpec.describe Admin::UsersController do
       expect(evil_trout.approved).to eq(false)
     end
 
+    it "will create a reviewable if one does not exist" do
+      evil_trout.update!(active: true)
+      expect(ReviewableUser.find_by(target: evil_trout)).to be_blank
+      put "/admin/users/#{evil_trout.id}/approve.json"
+      expect(response.code).to eq("200")
+      expect(ReviewableUser.find_by(target: evil_trout)).to be_present
+      expect(evil_trout.reload).to be_approved
+    end
+
     it 'calls approve' do
       Jobs.run_immediately!
       evil_trout.activate