Add more information to the 403 page if a user is not a DANDI user

[asmacdo]

If a user with a github account is not a DANDI user, when they attempt to login to the hub they will get a 403 without any helpful information.

This 403 page is part of the jupyterhub server, so its not clear if we can modify that. If not lets see if we can add upstream.

[kabilar]

Thanks Austin. To improve the DandiHub registration flow, I would also suggest that on the DandiHub landing page (hub.dandiarchive.org) we add a note that explains that users must first register (and be approved) for a DANDI account. This page may also be apart of the JupyterHub server, so we could put another website ahead of the Sign in with GitHub page with the above note.

[asmacdo]

This doesn't affect the landing page, but it looks like we could add a custom message with the 403.