feat: Add support for the Discussions MFE

[arbrandes]

Adds support for the Discussions MFE, enabling it by default.

Optional features also enabled via waffle flag:

• Learner's tab in the MFE
• Extended moderation reason codes
• Reported content email notifications to moderators
• Enable learner stats in the activity API

Requirements

• [x] https://github.com/openedx/frontend-app-discussions/pull/364
• [x] https://github.com/openedx/frontend-app-discussions/pull/363
• [x] https://github.com/openedx/cs_comments_service/pull/399
• [x] https://github.com/openedx/edx-platform/pull/31345

[arbrandes]

@regisb, I'm still waiting on the backports from master to Olive in cs_comments_service and edx-platform, but the tutor-mfe bits are good enough to review, and the MFE actually works... as far as I can tell. I still see fixes to frontend-app-discussions coming in (I had to whip one up myself), so we'll want to keep our eyes open to pick them up before release day.

[arbrandes]

@regisb, yes, definitely nightly!

[arbrandes]

Ok, I've tested this in nightly and olive, and it's looking good AFAICT. We're just blocked on the edx-platform fixes.

[arbrandes]

Rebased on the new runtime config support: good enough for nightly. Probably worth another glance, @regisb.

[arbrandes]

...aaand, we're in!