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>