models: Allow to unselect maintainer projects in user profile
The admin page for user profiles have a field "Maintainer projects"
to grant maintainer rights to an user. It is a list of available
projects. When a project was selected, it was impossible to revoke.
It is now possible to unselect by clicking while holding the "Ctrl"
key.
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com> Acked-by: Stephen Finucane <stephen.finucane@intel.com>