fix: add has_dismissed field

[katrinan029]

No description provided.

[iloveagent57]

I was chatting with Adam about this offline, here's a proposal for a slightly different approach:

1. IMO, I think has_dismissed would be better modeled as a new type of LearnerContentAssignmentAction rather than a field on the assignment itself, with a new action type like learner_acknowledged_cancellation.
2. You could have the credits-available view filter out cancelled assignments based on the presence of such an action.
3. We would then add a new view to the LearnerContentAssignmentViewSet with the DRF action() decorator, like def acknowledge_cancellation(...). That view would add the ack'd cancellation action for the assignment.

Let me know what you think!