Whenever the server returns true for the can-assign Webdav property of
a system tag, it means the current user is allowed to assign,
regardless of the value of user-assignable.
This commit brings the proper logic to the web UI to make it possible
for users to assign when they have the permission.
Instead of checking for admin perm, use interface method
canUserAssignTag and canUserSeeTag to check for permissions.
Allows for more flexible implementation.
