Resolve HTTP 400 errors related to SSO/OIDC

[stevejalim]

Sometimes the Wagtail admin pages will trigger a 400 Bad Request error, which goes away if re-attempted (eg page is refreshed, or the draft is re-saved). This does result, frustratingly, in lost work, and is hard to predict, so frequent draft-saving is the only current mitigation, and that itself is not ideal.

The traceback puts this down to the SSO integration code from Mozilla's OIDC app.

mozilla/mozilla-django-oidc#303 outlines the actual issue when I (@stevejalim) encountered this on an MDN Wagtail build, plus mozilla/mozilla-django-oidc#312 as a related issue and mozilla/mozilla-django-oidc#358 as a possible fix.

We can talk with SUMO about have had to deal with this, because mozilla/mozilla-django-oidc#358 suggests it should be a solved thing.

However, if we can't remedy it with OIDC-level configuration we could also consider:

• adding autosave to Wagtail in a way that automatically refreshes the OIDC session if need be
• adding JS to the wagtail UI that periodically triggers a session refresh (without the autosave)

[stevejalim]

Here's the Sentry issue tracking this https://mozilla.sentry.io/issues/4524518956/

[stevejalim]

Here's how it looks when it occurs - we could add custom HTTP 400 template to help explain things, but that's not much of a sticking plaster