Let site plugins register site schemas (DB)

This issue only needs to be tested in 1 device. Install the choicegroup plugin __ with offline support in a Moodle site. Please notice that the offline support hasn't been integrated right now. In a course, create a group choice. Make sure to allow editing the response (to make it easier to test). In the app, login in the site and open the group choice. Go offline. Answer the choice. Check that you see a message saying that there's offline data, and the app displays the choice you selected in offline. Leave the group choice and open it again in offline. Check that you see a message saying that there's offline data, and the app displays the choice you selected in offline. Go online and synchronize using the top-right menu. Check that the data is sent and the right choice has been selected (you can open it in web with the same user to verify the data has been stored in Moodle).