Allow more characters in username

[Jessica Tallon]

Currently usernames can only be [a-z0-9] I think they should also support utf-8 characters.

[Christopher Allan Webber]

I would love to see this happen in some ways but am also cautious about it. Two concerns:

• "tricky" characters: ​http://www.alanwood.net/unicode/general_punctuation.html especially zero width characters, RTL, etc?
• Currently, all usernames make nice slugs, like /u/foobar/

[Loic Dachary]

cwebber: please re-open if you think this requires more discussion. I'm assuming three years is a long enough pause ;-)