Don't exclude 'system' from groups in drop-down

Fixes an annoying bug noticed by @pwalczysko today, where the 'system' group is not shown in the drop-down of groups & users in web. So you can't switch to see the data of yourself or other users in the 'system' group.

This has been the case since 4.4.0 - it's just not an issue you notice unless you have data in the 'system' group.