fix(client): validate emails against IETF rfc5321

* email must have >= 2 part tld
* email can have [1, 64] byte local length
* email can have [3, 255] byte domain length
* email can be up to 256 bytes long

For more info, see http://tools.ietf.org/html/rfc5321#section-4.5.3.1.1

Closes #563
refactor(comment): fix the regex attribution link
Merge pull request #619 from mozilla/issue-565-email-length-limit

fix(client): validate emails against IETF rfc5321