Add login or back to project link to "Not authenticated" page

If you are logged in you should see the "Back to projects" link.

If you aren't either redirect the user to the login page (and redirect to the project on login), or just show a login link. I think the former is the better UX.

Start at the "Not authenticated" page and trace how project and login navigation are handled. The work is done when authenticated users can return to projects and unauthenticated users can log in and are returned to the project, as described in the issue.

Written by the indexing model from the issue text.