Fix returning from IdP verification on FirefoxOS

Background: sessionStorage can be wiped on FirefoxOS when authenticating with the IdP. We need to store a momento with dialog bootstrapping info for when the user returns from the IdP.

* store primary IdP verification flow info in localStorage with a nonce
* Store the nonce in window.name for FirefoxOS, sessionStorage for IE8.
* When returning from the IdP, look for nonce, load info from localStorage.

fixes #3572
Simplify the nonce generation. Thanks @jedp!
Merge pull request #3592 from mozilla/issue_3572_firefoxos_idp_verification

Fix returning from IdP verification on FirefoxOS