Backport of Users page lazy multiselect group dropdowns #1128 to stable9

Users page lazy multiselect group dropdowns

Instead of pre-rendering all multiselects with lots of group entries,
the current groups are now displayed as simple labels.
Behind the labels there is a pencil icon like for other fields.
When clicking the pencil icon, the dropdown will be spawned and will
open itself.
Upon closing of the dropdown, the label comes back with the updated
selection and the dropdown is destroyed.

Extra non-available groups also in list

Fix group sorting in user list group selection