Make user list links and add "< Users" button

References https://github.com/TryGhost/Ghost-UI/issues/56

- Only show the invited users section, of there are any
- Each current user item block is now a link
- Add the [< Users] button, which was previously commented out
Now that there is an ```if```-block around the entire loop, this ```else```-block can be removed since it will never happen