Move the implementation of Bug 19273 out of Torbutton

Bug #19273 (closed) (e.g., 2cce8efa) has a part in Torbutton.

We should move it outside of it, and possibly move it before other Tor patches.

Uplifting it would be lovely, but very unlikely (since it explicitly says to use only Tor or Tails).

Finally, the title is a bit misleading, we could reword it to say it's the external app warning dialog, rather than saying "prevent JS from hijacking", even though this was the original reason for that ticket.