Configurable TLD string check

Trello: https://trello.com/c/U8kZlXxU

This PR enables the TLDs of claimed domains to be checked against a whitelist of allowed TLDs.

Background:

In 2c9296436da0fd884626d3a544217c1fb2eb2a99 the check for valid domain names has been completely removed. We need a way to allow "test" TLDs but still check for valid domains. Also, we must not forget to reenable the TLD check for the `HIV` string on launch. This also acts as a documentation / reminder.