Include bridge configuration into about:preferences

Torbutton as a standalone extension is going away (legacy/trac#10760 (moved)) and while doing so we restructure our toolbar making it more usable by exposing New Identity directly on it (legacy/trac#27511 (moved)). However, we need to find a new home for the bridge configuration as well if we want to remove the onion button from the toolbar. The current plan is to move it to about:preferences as a general setting. This ticket tracks that work.