Node-solid-server: Check usernames for invalid characters at character creation

[stuartyeates]

Currently on https://inrupt.net/ you can register with a username of (for example) "stuart yeates" which then generates a "Welcome to Solid" email with a bad link of https://stuart/profile/card#me

Either the entire system needs to properly handle and encode non-alphabetic characters (probably fairly hard) or the input form needs to check and warn when non-alphabetic characters are present.

[kjetilk]

Indeed, we have to address quickly. Also reported in https://github.com/solid/solid/issues/157

[fourcube]

@kjetilk I'd be happy to work on this tonight.