feat(signin): Add cached signin
Merge pull request #1449 from vladikoff/login-chooser

feat(signin): Add cached authentication credentials
fix(hsts): force hsts headers and use milliseconds
Merge pull request #1615 from mozilla/issue-1614-hsts-header

fix(hsts): force hsts headers and use milliseconds

Fixes #1614
r=ckarlof
feat(client): Add support for `preVerfiyToken`.

`preVerifyToken` is sent by trusted RPs to indicate that a user is already verified. The Auth Server
then does a dance with the trusted RP to determine whether to create an account for the user without
the need to verify their email address. If the user is pre-verified, the signup flow completes at the
`/signup_complete` screen. If the pre-verification fails, the user is still requried to verify their
email address as is currently done.

This PR also introduces the concept of a "Relier" model. A Relier is a model
that represents an RP and information about/from the RP.

fixes #1540
Start to add functional tests for the preverified flow.