Adding OpenID returned mismatching email addresses to error screen. Fixes #186
Add Stateless Account Linking feature. Fixes #178.

* Adds new flow to id_mismatch
* App now sends email
* Adds second cookie
* Adds secret token for email verification loop
Remove dead code and interceptor. Tweak template.
Merge pull request #195 from mozilla/issue-178-stateless-account-linking

Issue 178 stateless account linking