Reject Phame domains which include a port number

Summary: Via HackerOne. This doesn't actually have any security impact as far as we can tell, but a researcher reported it since it seems suspicious. At a minimum, it could be confusing. Also improve some i18n stuff.

Test Plan: Hit all the error cases, then saved a valid custom domain.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Differential Revision: https://secure.phabricator.com/D8493