Allow hyphen in user name when creating from the UI

• When trying to create a local xwiki user with name "autx-edno" , it is validated "Ok."
• Upon saving, you receive error "Invalid username provided. Please use only letters from the latin alphabet, numbers, and the underscore character '_'."
• Contrary, when using LDAP authentication, usernames containing a hyphen are working fine. The users are created automatically on first login of the user.
  In fact, we have been running like this on xwiki 8.4.X for about 2 years now.

1. Since the hyphen does not seem to have any negative impact, it should be allowed within usernames
2. The validation should reflect which chars are really allowed

See also: https://jira.xwiki.org/browse/XWIKI-15055