Support offline manual completion

In web, login as teacher/admin and enable manual completion for a certain activity in a course (Edit settings > Activity completion > Students can manually...). In the app, login as a student and go to the course and section containing that activity. Check that you can change the completion in online. Now go offline and change the completion. Check that the completion is changed and you see a message in the activity saying that it isn't synchronized. Go online and wait a bit for the data to be syncrhonized (it can take up to 5 minutes). Once it is synchronized, the view should refresh and the completion should stay as it was in offline. Go offline again and change the completion. In web, change the completion twice (so it ends up as it was). The completion in web should be different as the one stored in offline in the app. In the app, go online and do a PTR or wait for it to synchronize. Check that you see a message saying that some offline completion was deleted. Go offline again and change the completion. In web, disable the completion of that activity (Edit settings > Activity completion > Do not indicate activity completion). You'll have to unlock the completion. In the app, go online and do a PTR or wait for it to synchronize. Check that you see a message saying that some offline completion was deleted.