Support durable invitation links to hunts

[jpd236]

Allow members of a hunt to generate invitation links to that hunt. Invitation links would allow existing users to sign-in to get access to that hunt, or allow new users to create an account and then join that hunt.

I think the most reasonable way to start here would be to allow only admins to (re)generate invitation links for a hunt, but allow all hunt members to view the link once generated. We could go further like Discord and track invitation URLs on a per-user basis, with an admin dashboard to view invitation usage and block individual invitation URLs in case of abuse, but this feels excessive to me for now.