This change was added to limit deleting of comments in standard groups
views to admins only, and admins and tutors in course groups. This commit
adds the functions
    User::can_moderate_view()
and
    GroupType::get_view_moderating_roles()
to implement this functionality.

Change-Id: Ifb8f4fc55d31fd7c5928c73c3d81710934bfdad8
Signed-off-by: Edward Murrell <edwardm@catalyst.net.nz>