Allow teachers to delete messages for all users within their conversations in the app

This issue should be tested once  MDL-65132 has been integrated. You only need to test this in 1 device. In web, make sure to create at least 1 group conversation with a student and a teacher. Assign the capability "moodle/site:deleteanymessage" to the teacher. In the app, login as a student and open the group conversation. Send at least 3 messages. Using the top-right menu, enable show delete. Click the trash icon next to any message (please remember which one it is). Check that the confirm message says the message will only be deleted for you. Confirm the delete and check that the message has been deleted for you. Now login as the teacher and open the same conversation. Check that you still see the message that the student deleted. Using the top-right menu, enable show delete. Click the trash icon next to another message (please remember which one it is). Check that the confirm modal contains a checkbox to delete the message for everyone. Click OK without enabling the checkbox. Check that the message is deleted for you. Now click the trash icon next to another message (not the same as in step 6, please remember which one it is). Enable the checkbox to delete for all users and confirm. Check that the message is deleted for you. Now login as student again and open the group conversation. Check that you see the message from the step 10 (it was deleted only for the teacher) but not the one from step 12.