feat(client): Allow TLD only domain names in email addresses

The email `user@com` is a valid email according to the IETF RFC5321. Marketplace allows them. We should too.

http://tools.ietf.org/html/rfc5321#section-4.5.3.1.1

fixes #1871
Merge pull request #1909 from mozilla/issue-1871-tld-email

feat(client): Allow TLD only domain names in email addresses