Gitea allows to whitelist email domains so that only email addresses from certain domains are allowed to register an account, but does not currently allows to do the opposite: blacklisting email domains so that addresses from certain domains are *forbidden* to register an account.
The idea has been briefly mentioned in the discussion about issue #6350, but never implemented. This PR does that.
The rationale is that, in my experience of running a Gitea instance, *a single email domain* is responsible for *most* of the spam accounts, and for *all* of the spam accounts that manage to get past the email confirmation step. So on top of the other spam mitigation measures already available (email confirmation, CAPTCHA, etc.), having the option to block a particularly annoying domain would be helpful.