Bug 347213: When moving a bug to another product, Bugzilla should let us preemptively put the bug into a (valid) group which is not a default one for this product

Bug 303183: Bugzilla fails to warn when a product change would force the removal of an optional group

Patch by Frédéric Buclin <LpSolit@gmail.com> r=mkanat a=LpSolit