Disable web search in the Unsafe Browser's address bar

Originally created by @anonym on #12540 (Redmine)

Original feature request: https://mailman.boum.org/pipermail/tails-support/2017-May/001330.html

The user wants to avoid leaking mistyped URLs to the default search engine (Google) e.g. your LAN’s router’s IP address.

Implementation: set the pref keyword.enabled = false.

Feature Branch: bugfix/12540-disable-search-in-unsafe-browser