Modify the setup wizard and Network Settings window to allow automated
retrieval of bridges using Moat, a BridgeDB service which works
over a meek transport and requires the user to solve a CAPTCHA to
obtain bridges.

The new tl-bridgedb.jsm JavaScript module handles all communication
with BridgeDB, and it functions by starting a copy of
meek-client-torbrowser and operating as a PT client parent process
(see https://gitweb.torproject.org/torspec.git/tree/pt-spec.txt).

This feature can be disabled (and the Moat-related Tor Launcher UI
hidden) by setting the pref extensions.torlauncher.moat_service to
an empty string.