feat(client): Add support for `preVerifyToken`.

`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.

issue #1540

Items that still need to be completed:
* [x] - Update the fxa-js-client to pass along `preVerifyToken` - See https://github.com/mozilla/fxa-js-client/pull/127
* [x] - Update the fxa-auth-server to handle `preVerifyToken` - See https://github.com/mozilla/fxa-auth-server/issues/780
@ckarlof - I coded it up this way initially, and then changed the behavior after talking to @johngruen and @ryanfeeley. Their feedback was "the user should still have to click on the link." It's an easy change either way. @johngruen and @ryanfeeley, could you weigh in?

Current Behavior:
A pre-verified user sees the "signup complete" screen and must click on the link in completion screen to redirect back to the RP.

Proposed Behavior:
After setting a password, a pre-verified user never sees the "signup complete" screen, instead they are redirected to the RP without further interaction.