Make username validation compatible with fbgive

[deckar01]

var validLength = value.length >= 3 && value.length <= 255;
var validEmail  = validator.isEmail(value);

https://github.com/stellar/stellar-wallet/blob/master/lib/util/validate.js#L92

• This does not properly validate that use username is at least 3 characters, because the "@stellar.org" has already been added.
• The max length should certainly not be 255, because this allows usernames that are valid addresses.

I think we should stick with the validation in ix. I don't see any reason that the username should have to pass the RFC spec for email addresses. The rules are too arbitrary given our restrictions and too hard to explain to users when they are violated.

/^((([a-z]|\d|[!#\$%&'\*\+\-\/=\?\^_`{\|}~]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])+(\.([a-z]|\d|[!#\$%&'\*\+\-\/=\?\^_`{\|}~]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])+)*)|((\x22)((((\x20|\x09)*(\x0d\x0a))?(\x20|\x09)+)?(([\x01-\x08\x0b\x0c\x0e-\x1f\x7f]|\x21|[\x23-\x5b]|[\x5d-\x7e]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(\\([\x01-\x09\x0b\x0c\x0d-\x7f]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF]))))*(((\x20|\x09)*(\x0d\x0a))?(\x20|\x09)+)?(\x22)))$/i

https://github.com/chriso/validator.js/blob/master/validator.js#L38

1. A username is composed of both a user and domain parts, separated by an "@"
2. Each part of a username must be between 3-20 characters, and validate against the regex: "/^[a-zA-Z0-9]+([._-]+[a-zA-Z0-9]+)*$/"

https://github.com/stellar/ix/issues/410#issuecomment-65481036