Remove rack-cache + add fastly.

We spawn a thread to purge the Fastly cache when an issue is updated.
Force SSL through Fastly (in staging)
Avoid redirect loop by respecting Fastly's custom SSL header.

More info: https://fastly.zendesk.com/entries/23396536-How-do-I-tell-if-this-is-an-SSL-request-or-not-
Try another approach.
Translate Fastly's custom SSL header to something Rack can understand using middleware.
Use custom session controllers.

We were using Devise::SessionsController for two resources - /{user,representative}/sign_in.
This confuses `force_ssl` which does a plain `redirect_to` with no specific controller.
Fix redirects for HTTPS
Fix mixed content warnings for Gravatar images.
Make sure session cookies are secure if SSL is enabled.