Add feature to provide custom link to user guide / ability to hide the link

[wmcclin]

Currently, the User Guide links to our generic user and admin documentation on Gitbooks. Some projects have their own custom guides. In the case of MPATT (https://www.seasketch.org/mpatt), they've added a user guide to the "About" page. To avoid confusion, perhaps it would be good if an administrator could show/hide the User Guide button and/or provide a custom URL so that that could direct users to their own guide.