Create new functions to add members, invite members, and decline
membership requests for an institution.  These functions check
appropriate institutional admin permissions and are callable by
the webservice api.

Also modifies the existing bulk insitution membership page to make
use of the new functions.

Change-Id: I52aced5bf398835c8590bb2bab8a7028335af01b
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>