Harden node viewset to ensure missing data sets it incomplete

[bjester]

Observed behavior

There seemed to be a gap in our validation and viewset logic that allowed a node to be marked as complete, even though it did not have proper mastery criteria. This resulted in a failure during publishing
#4163

Expected behavior

We harden the content node viewset to ensure that completion criteria sets the node incomplete when it's invalid.

User-facing consequences

Users don't have insight within the UI regarding which nodes are incomplete and won't be published

[rtibbles]

Should make sure we are leveraging the mark_complete function here: https://github.com/learningequality/studio/blob/unstable/contentcuration/contentcuration/models.py#L1810, and that it is properly applying any updates (it's possible that the slightly over complicated logic in the viewset/serializer logic is not passing complete as a field to be saved).