Tor Launcher's bridge settings page doesn't show the help button when there's no provided bridges

The summary says says it all. Clearly the bridge help is equally (actually I'd like to say more) important when there are no provided bridges. This problem is in version 0.2.5.2 or git @ commit 773de03.

See the bridge_help branch on git://git.tails.boum.org/tor-launcher for a fix. It's quite simplistic, and perhaps more care should be given to where the button is placed; the button's position in my fix may give the impression that it's only a help for the custom bridges case.