Show a "Signing in" message after the user selects an email from the email picker or successfully authenticates.

* Give the same signin notice for both primary and secondary sign ins
* Replace primary_user_verified.ejs with signing_in.ejs

fixes #3708
Detanglement of generate_assertion from complete_sign_in.

* The stuff dealing with the animations for completing sign in is now in its own module, complete_sign_in.
* Clean up elsewhere. Add new tests.
Move the closing animation code from dialogHelpers into complete_sign_in.js
* Coherent parts should go together
* To dom-jquery add setStyle and getInnerWidth
Fixup a few comments
Merge pull request #3721 from mozilla/issue_3708_signing_in_notice

Show a "Signing in" message after the user selects an email from the email picker or successfully authenticates