Fix group user removal from admin area

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Add tests for remove group member feature in admin area

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'fix-group-member-remove' into 'master'

Fix group member remove

Fixes https://github.com/gitlabhq/gitlabhq/issues/8090.

See merge request !1196