Allow preventing registration of certain localparts using regexes or globs

This issue has been migrated from [#13908](https://github.com/matrix-org/synapse/issues/13908).

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**Description:**

There are quite a few very obvious mxids containing racist slurs and other problematic content. It would be nice that _if_ you really have to have public registration, there were an easy way to deny registration of some problematic MXIDs based on a configured list of blocked IDs. While this isnt bulletproof either (no spam protection is) it will at least prevent the very obvious ones from being able to register.