Improved Group Member Management on User Administration

Allows for a quick and easy group membership management on the
user-administration page. Uses the select2 UI component to
autosuggest other groups, remove existing ones and lock in automatic
groups.
renamed
use computed
move available groups loader into router
move ajax into models
make jshint happy: missing semicolon
Merge pull request #2537 from ligthyear/group-member-management-on-user

Improved Group Member Management on User Administration