Fix primaries 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.
Merge pull request #3461 from mozilla/b2g-primaries

Fix primaries on FirefoxOS!