Add threshold field validation and properly handle updates to mastery models in all scenarios

[rtibbles]

Summary

Description of the change(s) you made

• Adds some simple validation for the threshold field to ensure it is either a string, integer coerceable or dict.
• Ensures that updates to the threshold key of completion_criteria can be made granularly when threshold is an object
• For example, just updating the m value of an m of n mastery model
• Ensures that partial updates to extra fields that contain the old way of encoding mastery model (at the top level) successfully write the entire representation into the completion criteria format.
• Adds regression tests for both scenarios