[server] Implement magic link for non-user group invites

[Jdyn]

The link should contain pre-filled information about the user that was given by the creator of the link.

fields:

• identifier, email or phone
• name, full name or potentially just first name

User Story

• A person receives an email invite that contains a link.
• The link should contain an encoded hash
• After clicking the link, the user is taken to a page, with loading state, "Fetching your invite!".
• the encoded hash is sent to the server and decoded against an existing invite.
• If there is a match, the prefilled information is returned, and the form appears with the data.
• the user fills out the form entirely
• Finally, the encoded hash is sent again along with the form.
• the user is created and automatically added to the inviting group.