one catalog item

[sverhoeven]

Can we make one catalog item with the following questions?

1. Do you want to use dcache or a samba server?
2. Username of grader?
3. Which student accounts should be created?

The catalog item should always install the nbgrader stuff.
The catalog item should not have terria js and experiment launcher.

This should merge #162 and #155 long running Pull requests into main.

If you pick dcache

1. The macaroon set in the catalog item should be valid
2. During workspace creation you have to supply a storage element for the dcache cache disk

If you pick samba server

1. Create samba_password secret in SRC
2. You should setup the samba server in SRC before creating a ewatercycle workspace.