Have sysops add and remove users from the 'gwtoolset' group

Let's keep the status quo for the time being; the Wikimedia
Commons community can decide to change this particular
setting in an internal discussion, should it be the case.

This is a follow-up to I4d9097e.

Bug: 58607
Change-Id: I33c68abb10517cb871956b25603920c3785f8762