email_hint - initial sketch
use hint to pre-fill email address in dialog
Finish hooking up email hint.

* Hook up the email hint to the email picker.
* Fix issue when calling authenticate after the email picker and the user must authenticate to the fallback IdP.
* Add tests for the email hint to both the email picker and the state machine.
Validate the emails coming in for experimental_emailHint
Merge pull request #3843 from mozilla/email_hint

Email hint