MDL-45965 assign: add notification capability

MDL-45965 assign: add notification capability

Adds a new capability that adds flexibility to what users receive grader
submission messages.  Includes phpunit tests.  Function is based off of
get_graders() but is separate because it is bad form to have a
capability that is dependent on another capabilities setting.
Merge remote-tracking branch 'origin/MDL-45965_master' into MDL-45965_master

Conflicts:
	mod/assign/locallib.php
	mod/assign/tests/locallib_test.php