Domains: more robust form

[stsewd]

This form was even accepting spaces...

Based on the regex from https://api.cloudflare.com/
(pattern: ^([a-zA-Z0-9][\-a-zA-Z0-9]*\.)+[\-a-zA-Z0-9]{2,20}$)

Closes #1707

[humitos]

Based on the regex from api.cloudflare.com
(pattern: ^([a-zA-Z0-9][\-a-zA-Z0-9]*\.)+[\-a-zA-Z0-9]{2,20}$)

This should probably be a comment in the code next to the regex. With this link, that goes directly to the pattern: https://api.cloudflare.com/#zone-create-zone

[agjohnson]

Looks great. Noted what looks to be a removal of unused code. It might be worth double checking we weren't using the attribute on commercial/etc. Based on this PR it looks like the removed code was ancient and wasn't even used.