Non-student behavior

[gabrc52]

As a bare minimum, I have done 4b5bbd8 and sipb/uplink@0467357 which does the job but the UX in this is bad.

We should:

• We could consider simply deleting this wall. It's just that part of the appeal of the Messenger chats was that they were student-only.
• Give a good error message when people are denied entry.
• Offer something with faculty and staff in mind, since they would want to create chats too. For now, they can create their own chats manually.
• Check the affiliation from this web app too, not just Matrix. It's hard since it requires having signed up or querying LDAP
• The affiliation from Touchstone/SAML (which presumably does come from LDAP) may be incorrect. Does it really take into account cross-registered students and all?