Fixes#6640Fixes#6866
This pull request adds support for our new Autograph signing server, adds it to our dockerized services and adds tests that now actually call our signing server.
* Add a waffle-switch for calls to our new autograph signing server
* Add the autograph signing server to docker-compose
* Add the autograph signing server to our travis setup
* avoid us specifying the signing server settings everywhere in the code, one single source of truth
* more use of `responses` instead of mocking, leaning towards
the future where we can actually hit a signing server instead
of a mock
* disallow http requests generically, unless configured otherwise
* Add migration for waffle flag