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.

fixes #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
* [x] - Update 123done to generate `preVerifyToken`s & rip that cruft out of here. See https://github.com/mozilla/123done/pull/114
* [x] - Update the auth-server's default local.json config to allow `preVerifyToken` from 123done. See https://github.com/mozilla/fxa-auth-server/pull/794
* [x] - Update the functional test to make use of 123done instead of generating tokens locally.
* [x] - Update the /oauth/signup page to redirect to the RP if preverification was successful.
Might it be more self documenting to have separate parameters for each possible dependency? At the very least we should probably document what could be passed in to `deps`.