Allow hub administrators create a "sign up link" that lets users click and create accounts automatically

[choldgraf]

Context

For one-off workshops or events, it can cause friction to ask hub administrators to ask for credentials of each user they want to join the hub. For example, if an even doesn't know who will attend ahead of time, but wants to provide all attendees access to a hub. This was recently noted in the Jack Eddy Symposium notes by @colliand.

Many web services have the notion of a "magic link" or "sign-up link". For example, Slack will let you create a link so that when anybody clicks it, they are immediately given access via sign-up window. Here's an example of in Slack:

Proposal

We should explore allowing a hub administrator to generate a link like this via the Hub Admin panel. When a user clicks it, some process to auto-authenticate should happen so that they can log-in immediately.

Updates and actions

No response