Patch XSS vulnerability in Discussions

This ports the upstream patch [1] into our fork. This change is made
manually, instead of cherry-picking, as we currently lack the most
recent changes to the discussions product [2].

We will pick up the recent changes (and patch tests) the next time we
merge from upstream.

[1] https://github.com/edx/edx-platform/commit/58553a7fb8fbc33dfd40c8e7ca39b0f896fb5b32
[2] https://github.com/edx/edx-platform/commit/988e4e6da5bf5f8db1cb269a82cc272c6d73ed72