Fix a bug when adding 0 member to course using API
Update guard plugin
modified groupMembers and enrolment in the api

For groupMembers, I have changed the reaction to adding a group
member that is not in the course to skipping the user from terminating
the request. I have also added the task of removing members that
are not in the new list.

for enrolment, I have changed the reaction to enroling users that
do not exist to skipping the user from terminating the request.
Similar to groupMembers, users already enroled in the course but do
not appear in the new list would be removed.

I have added tests for the api to test the changes mentioned above.

Conflicts:

	app/controllers/v1_controller.php
	app/tests/cases/controllers/v1_controller.test.php
fixed a bug in the api

The api has a small bug for enrolment where the classlist may not
be complete if any of the ids in user_tutors, user_courses, and
user_enrols tables are the same. This has now been fixed by
using array_merge instead of the union operator (+).