User-bound instant invites

[Zoddo]

It would be useful to be able to create single-use invites that are only acceptable by a specific user.

For example, this could be implemented by adding a recipient_id field to the Invite Object and Create Channel Invite endpoints.
When such an invite is used, the API should check if the current user matches recipient_id and otherwise reject the request.

Why is this needed?

This could be used by apps as an alternative to the guilds.join OAuth2 scope if they want to secure an invite to a server.

This has a few advantages:

• the user doesn't have to approve access to their account through OAuth2
• it's more straightforward for the average user because joining such servers is exactly like joining any other server (approving an oauth2 scope that "give access to your account" can be scary to non-advanced users)
• the user needs to do a positive action to join each server they are invited to (whereas with the aforementioned scope, after approving access to their account, the app can join the user to additional servers without explicit and positive consent)
• this could be a step towards making guilds.join a privileged scope to stop abuses around it

[erkinalp]

Suggested API endpoints for the function:

• POST /channels/{channel.id}/invite/{user.id} — invite user
• DELETE /channels/{channel.id}/invite/{user.id} — disinvite user
• DELETE /channels/{channels.id}/invite/@me — refuse a user-specific invite directed towards self
• GET /users/@me/invites — get own invites
• INVITE_USER event
• GET /users/{user.id}/invites — get own invites sent to a certain user ID and invites sent by user_id to self

[Zoddo]

I don't think we need to add such complexity here. The current invite system works well, we just need to be able to restrict a (one-use) invite code to a single user.