This PR adds `Cache-Control: no-cache, no-store, must-revalidate` header to the response on discussions. This prevents browsers and intermediary proxies from serving stale content when browsing the forum.
*Sandbox URL*: TBD - sandbox is being provisioned.
1. Navigate to the forum on a course and open up the Developer Tools in your browser.
2. Check the `forum/` response headers for `cache-control`
[ ] @xitij2000
[ ] edX reviewer[s] TBD