Blacklists and Whitelists feature for Admins and Organizers

This is to keep track of a feature which I think could benefit organizers and is important for site admins.
We need a general blacklist for emails and usernames. It can help us fight against domain names which are known for spam, as well as make it easier for us to ban users that cause trouble on the platform.

For Organizers, it can help them control who has access to their competition, either because they don't want to manually approve everyone, while needing a way to ban specific users by using a blacklist, or create a whitelist with wildcards so that only a specific domain can sign up to their competition(s).
Here is an example where a blacklist can be useful for both organizers and admins : https://github.com/codalab/codabench/issues/1733

Here is an example of why a whitelist with wildcards can be useful for organizers or site admins : https://github.com/codalab/codabench/issues/1827

Both the Blacklist and Whitelist should be able to use Regex and wildcards to be as flexible as possible, allowing admins and organizers more control on their instance and competitions.

No files, tests, or entry points are identified in the issue. Start by locating the existing competition signup and user-ban or approval flows, then clarify the administration and organizer scope, matching rules, and acceptance criteria before implementation.