Add prefetch and offline grading features for assignment

To test that, use an assignment with simple grading, outcomes and feedback comments set up. Grade assignment submission in offline or server down. Go back to the main screen of the assingment and confirm that you see a sync warning message. Confirm there is a Sync link on the context menu. Clicking on it in offline/server down shows an error. Go back online or turn server up and click synchronize. Confirm that grades have been stored. Grade assignment submission again in offline. Before synchronizing, go to Moodle web and grade it in there. Now go back in the app and click sync in online. Check that you see a message telling that offline grades have been discarded. Finally, grade assignment submission again in offline, go back online and wait for sync to trigger (could take more than 10 minutes). Check that grades are automatically sent. Grade the assignment on the web using the gradebook and sync later: Using the app (OFFLINE). Grade numerical, outcomes and feedback. Save, so they will be stored offline. Using the website: Navigate to the course, and use the grades link on the navigation block. Grade an outcome from website. (Use different selection from the previous on step 1). Back in the app. Go online and sync. Check in the app and on website that the numerical grade and feedback are the ones you entered on step 1. Confirm that the outcome is the one you entered on step 4. Repeat steps 1-3. Grade with numerical grading from website. (You'll need to check the override checkbox on gradebook for the row you are setting). Use different selection from the previous on step 1. Back in the app. Go online and sync. Check in the app and on website that only feedback is being synced. Outcome and numerical is not saved because they are overridden. From the app, grade and save again, grade and outcomes won't be saved. Uncheck and save the "Overide" checkbox you set on previous step. From the app, grade and save again, grade and outcomes will be saved. Note: From the app we cannot know if grades are overridden (and locked).