Disable caching in forum home

Description

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.

*Testing instructions*:

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`

![image](https://user-images.githubusercontent.com/5691347/63456036-6bebe800-c424-11e9-9e5c-e65d1cbc5028.png)

*Reviewers*

  • [ ] @xitij2000

  • [ ] edX reviewer[s] TBD

Status

Assignee

David Ormsbee

Reporter

Open Source Pull Request Bot

Labels

Contributor Name

Paulo Viadanna

Repo

edx/edx-platform

Customer

Epic Link

None

OSCM Assignee

None

Priority

Unset
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