Add a preference to disable OnionAlias

At Tails, we use Tor Browser in two different ways:

• the Tor Browser
• the Unsafe Browser, which actually uses the clearnet.

For the Unsafe Browser, the OnionAlias feature makes no sense, because it cannot resolve onion addresses anyway. Not only that, but having it means that just opening the Unsafe Browser makes a DNS query to securedrop.org. This makes it very easy to fingerprint a Tails user (if they are using the Unsafe Browser, at least).

Therefore, we'd like to have a preference in prefs.js that disables this mechanism altogether.