Validation on `learner_managed` may fail to save changed M/N values on imported exercise

[bjester]

Observed behavior

Split out from #3757

I import and exercise from KA EN channel that has 5/7 goal, and change it to 3/5, import into Kolibri, but on the learner side I still see the Get 5 correct. Same thing for exercise imported from the CK12 channel, default was 5/5, I lowered to 3/5, but no change in Kolibri.

From Blaine: testing this, I found that it works correctly, but the change to the M/N values may fail to save if combined with an update for invalid data like to learner_managed

Expected behavior

learner_managed should default to false and avoid causing a failure

Errors and logs

{
  "disallowed": [],
  "allowed": [
    ...
  ],
  "changes": [],
  "errors": [
    {
      "key": "fcce9b50254248fb82ca7ec1c15ed40a",
      "mods": {
        "changed": true,
        "extra_fields.options.modality": null,
        "extra_fields.options.completion_criteria.threshold.n": 9,
        "extra_fields.options.completion_criteria.learner_managed": null
      },
      "errors": {
        "extra_fields": {
          "options": {
            "completion_criteria": {
              "learner_managed": [
                "This field may not be null."
              ]
            }
          }
        }
      },
      "server_rev": 4520,
      "table": "contentnode",
      "type": 2,
      "channel_id": "5fbe14ce253e4f22ab31019c6c4e4401",
      "user_id": null,
      "created_by_id": 2512
    }
  ],
  "successess": [],
  "tasks": [],
  "successes": []
}

Additional information

Steps to reproduce the issue

1. Import a KA exercise into a channel
2. Update the exercise's M/N values
3. Publish the channel
4. In a Kolibri instance, update the options.ini > CENTRAL_CONTENT_BASE_URL = https://hotfixes.studio.learningequality.org
5. Import the channel into Kolibri
6. Open the Coach planner, and add a new quiz
7. Browse for the exercise
8. Ensure the completion description appropriately captures the M/N criteria

[rtibbles]

Fixed in #3757