Restrict registration on an instance to allowed email domains

Problem

We currently run a closed community on Discord. Every member is verified with a custom bot to have a specific email domain using email verification. This is currently not so easy possible with Fluxer.

Proposal

As I can configure a Fluxer Instance to only host one community it would be nice to configure an email whitelist, too. This way a Fluxer instance could be further customized to only serve a specific usecase. You could configure a whitelist of email domains in the admin panel to only let people in with a specific domain which then gets validated using email confirmation.