When deciding who can moderate comments on a group view, the
'moderating' roles are stored in the view object, but they don't
need to be, because they only depend on the group settings.  So
we can remove moderating roles from the View object, and get them
directly using the group id when required.

Change-Id: I69f5cd467e474a1fd9c0c589322c6a3935aa3482
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>