Support completion criteria for content nodes

[bjester]

Description: TODO

Related #3078

Schema

{
 "completion_criteria": {
 "threshold": mixed | null,
 "model": "time" | "approx_time" | "pages" | "mastery" | "reference" | *,
 "learner_managed": bool,
 }
}

Examples:

Time

{
 "model": "time",
}

Approximate Time

{
 "model": "approx_time",
}

Pages

{
 "threshold": 2, // count
 "model": "pages",
}

Mastery

{
 "threshold": { ... }, // mastery data
 "model": "mastery",
}

Reference

{
 "model": "reference",
}

[rtibbles]

Made a couple of updates to the spec above to handle the approximate time labelling that we need, also deferring to the 'duration' top level property on content nodes for time based thresholds.

[rtibbles]

Constants for model added to le-utils here: learningequality/le-utils#98