fix: dont serialize Submission.first_two_pages through /api/v1.

[rjsparks]

Fixes #4083

[jennifer-richards]

Looks good. My only concern was whether the makeresources management command will interact with this. I think it will ignore models that already have resources even if those differ from what it would generate, but thought I'd raise this just in case my understanding is incorrect.

[rjsparks]

dev ➜ /workspace (no2pages@api ✗) $ ietf/manage.py makeresources submit

Inspecting ietf.submit ..
  nothing to do for ietf.submit