Bug 1597133: Adding institutions to groups that don't have them

And adding an institution selection to the Admin -> Group search

Also making sure that we can retain supplied shortname or throw error
if there is a clash

behatnotneeded

Change-Id: Ibbd6c05c1428e522c048b10c64389ebe3360473a
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>