[stable6] Show warning page when accessing server from an untrusted domain

Added early check for the requested domain host and show a warning
page if the domain is not trusted.

Please review @karlitschek @LukasReschke 

I haven't added localhost yet.
Should I add it:
1) Hard-coded in isTrustedDomain(), making sure to allow "localhost", "127.0.0.1", "localhost:8080", etc
or
2) Auto-add it to the config.php when the migration is run. Doing it here might miss people who already migrated, and I don't really want to add yet another code block with hard-coded version numbers to add these additionally

If you think that 1) isn't an issue I'll go with that approach.