Move Gopher back into core

In Mozilla 2.0, Gopher support was removed from core with the promise that "Gopher will be an option as long as people are motivated to own its implementation."[1]. This was done, thanks mostly to the good work of Cameron Kaiser[2]. In fact, OverbiteFF actually improved on the already quite good support that was there before. For a time, everyone was happy, or at least no one was excessively unhappy.

However The new WebExtensions API cannot, by design, possibly implement a Gopher protocol handler. As Gopher was promised to remain an option, and the possibility of implementing it as an extension is being removed, it'll have to move back into core.

[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=388195#c47
[2]: http://gopher.floodgap.com/overbite/