Fix primaries on FirefoxOS!

@jedp - this is a PR to fix the "relay frame cannot be found" error on FirefoxOS

* Save a marker in sessionStorage that indicates the return from a primary is in a native context. If so, do not attempt to set up the channel.
* Add the ability to redirect to a site from the dev menu. This is useful on FirefoxOS where the trusted UI has no URL bar.