mod_block_registrations: Support blocking confusable usernames (UTS 39) This prevents registering e.g. "admin" but with a Cyrillic 'a' U+0430 and other such variations, including keming like "adrnin".

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```