Added Whitelist of e-mail domains

[cydh]

• Added config UseWhitelistEmailDomains and its file config/whitelistemaildomains.php
• Changed AllowDuplicateEmails from boolean to integer to allowed duplicate e-mail by n numbers. 0: No duplicate allowed, -1: ignore duplication check

• [ ] Community test
• [x] Add populer domains in whitelistemaildomains.php

[cydh]

This is a neat list from nice project https://github.com/mailcheck/mailcheck/wiki/List-of-Popular-Domains

[hurtsky]

why not implement the mailcheck.js xD

[Everade]

This would block out all personal domains, and the current list is still lacking a very substantional amount of popular ones for several countries. I think a blacklist would make more sense to get at least rid of trashmails, or a service implementation that offers more settings.

[Akkarinage]

I agree, a blacklist would make more sense in this scenario.