add reverse-proxy for CSP violations

[warner]

When a Content Security Policy violation report URL lives in the same origin as the page which included the CSP header, the report will contain more information, which can be very helpful to track down what exactly is causing the problem. We're currently sending violation reports to a separate server (and will probably continue this in the future). To do both, we need to configure a reverse proxy, so CSP reports from e.g. https://accounts.stage.mozaws.net/signup can be reported to https://accounts.stage.mozaws/net/_/csp-violation and then get forwarded to https://fxa-csp-violations.stage.mozaws.net/_/csp-violation. We should probably build this as an nginx rule, but it could also be done with a route in the node.js server.

In stage/production we use the nginx configuration to deliver the CSP header anyways, so it's a good fit to use the nginx config for the reverse proxy too: it's easier to keep the header and the proxy in sync by having them live in the same config file.