Add stricter username rules:
* Only allow usernames to contain basic letters, numbers, CJK characters, underscores, dashes and periods.
* Don't allow names to start or end with punctuation.
* Don't allow names to have multiple underscores in a row.
* Don't allow active users to have names that look like deleted users (e.g. "user_1234").
* Don't allow emoji or any other Unicode characters except for Chinese, Japanese, and Korean
characters. CJK characters are currently grandfathered in but will be disallowed in the future.
Users with an invalid name will be shown a permanent sitewide banner until they change their name.
The median username length is 8 characters. The 99% percentile is 18
characters. The 99.9% percentile is 24 characters. About 750 users have
a name more than 24 characters long.
This doesn't do anything about existing users with long usernames.
Note that this is the length in Unicode codepoints, not grapheme
clusters. Some Unicode characters and emoji may be a single glyph but
composed of multiple codepoints.
