Add support for multilingual countries (like Switzerland)

Issue Summary

The current language detection and switching by hostname options are not sufficient for countries like Switzerland. In Switzerland there are three languages spoken, German, French and Italian.

ATM, I'm able to enter one hostname for each language. For all three languages the setting is already occupied - domain.de, domain.fr, and domain.it. I've no chance to setup the language for domain.ch.

We need to re-organize Translatable's settings:

1. Add button to add a hostname (domain)
2. For each hostname (domain) present a radio button list:
   • Detect by browser locale
   • German
   • French
   • Italian

For this purpose, configuration of hostnames has to be moved to a new tab "Hostnames".
Additionally, the "Switch by hostname" setting needs to check if all enabled languages have been assigned to hostnames. If not, that option cannot be enabled.