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.
The notification tests were not restoring the clock properly, but
indirectly helped other tests pass.
Since now we're restoring the clock properly, the other tests were fixed
to still work.
For admins: display the namespace behind the tag name.
For users: no namespace, don't display non-assignable tags in the
dropdown, display already assigned non-assignable tags with a different
style