Allow admin registration links to automatically join a specific guild

On a self-hosted instance with registration set to "Closed", users without an existing account cannot register through an invite. This applies both to invites for a community/guild itself and to invites that point to a specific channel within that community. Instead, they are shown: “Registration is currently closed. Use a registration link from an admin to create an account.” An admin registration link allows them to create the account, but it has no connection to the community or channel they were originally invited to. This means onboarding a new user currently requires two separate links:

1. an admin registration link to create the account
2. the original community/channel invite, which has to be opened afterwards

On a self-hosted instance with multiple communities, this makes the onboarding flow unnecessarily awkward.

Allow an admin registration link to optionally target a specific guild/community. When creating the registration link in the admin panel, the admin could optionally select a guild. If the link is used to create a new account, that account would automatically join the selected guild after registration. If the registration originated from or is associated with a specific channel invite, preserving that channel as the destination after signup would also make sense. If no guild is selected, the registration link would behave exactly as it does today. This would keep registration fully controlled by the instance admin while making the onboarding flow a single link instead of requiring a separate registration link and guild/channel invite. Regular guild or channel invites would not need to bypass the Closed registration policy themselves.

This came up while discussing closed self-hosted instances on the Fluxer development server. A “directed” admin registration link was suggested as a possible way to keep registration under admin control while avoiding the current two-link onboarding flow. The intended flow would be: Admin creates registration link for Guild A → user opens link → creates account → automatically joins Guild A. Optionally, if a specific channel is associated with the registration link, the user could be taken to that channel after joining. This is not intended to make normal guild or channel invites bypass the Closed registration policy.