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
pre-verify tokens work end to end!

* Add functional tests for the preverified flow.

To test using 123done, apply the following diff to fxa-auth-server/

diff --git a/config/dev.json b/config/dev.json
index 7919bd9..f08a03a 100644
--- a/config/dev.json
+++ b/config/dev.json
@@ -19,5 +19,6 @@
   "bounces": {
     "region": "us-east-1"
   },
-  "customsUrl": "none"
+  "customsUrl": "none",
+  "trustedJKUs": ["http://127.0.0.1:8080/.well-known/public-keys"]
 }

Then visit:

http://127.0.0.1:8080/api/preverify-signup?email=<your_email>
Merge pull request #1606 from mozilla/issue-1540-preverified-emails

feat(client): Add support for `preVerifyToken`.

A 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 required to verify their
email address as is currently done. See https://github.com/mozilla/fxa-auth-server/blob/master/docs/api.md#preverifytoken for further information on preVerifyTokens.

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
r=ckarlof,nchapman