Load static assets from S3 directly

[nimalank7]

Description:

• Static assets (such as CSS, images and Javascript) are stored in an S3 bucket. Currently to retrieve a static asset Fastly will make a request to an nginx pod running in router. nginx will then retrieve the static asset from S3 before passing it to Fastly.
• This commit makes Fastly talk directly to S3 without going through nginx and Router by are handled in the if block above which triggers a restart causing Fastly to poll the S3 mirror instead.
• If the static asset doesn't exist in S3 (e.g. www.gov.uk/assets/non-existent.jpg) then S3 returns an XML document and 403. Adding the condition to display a prettified error page for this would make the VCL code difficult to read as well as a special case to handle integration which has no mirrors. Since this error can only be seen if someone looks in the Chrome dev tools it's agreed that it is permissible to just return the S3 403 error instead.
• See the diagram in Fast path for Rails assets #73 for the previous method and the proposed new implementation

[sengi]

Do we need OPTIONS? Usually that's only for CORS preflight requests (when a client checks before sending a large POST/PUT request) but that wouldn't apply here since we're just serving static files and not handling uploads.

[nimalank7]

Yes you're right. We don't even need the Access-Control-Allow-Methods nor the Access-Control-Allow-Headers as these are only set with CORS preflight requests which we don't do as we're only doing simple GET requests.