Add blocklist of account name substrings

[nirik]

Downstream report:
https://pagure.io/fedora-infrastructure/issue/10743

This is asking for a way to prevent users from making accounts with 'fedora' in them, but I think this might be more generally useful as well if we could implement a list or file of them.

So, basically a file or config key that lists substrings that are disallowed in account names.