Rework Task Definition (within Course)

[eriq-augustine]

Right now, each task for a course is defined as a list of tasks of the specific type. I don't like this because:

• Most people will only have one of each type of task (at most) and the list can be confusing.
• We have to aggregate tasks on validation into a single collection.
• We have to add a reference to each type of task we add into the Course struct.

Instead of this, I would like to just have a list of tasks. Each item can have values like type, when, and options, and we can validate by having task types register themselves.

...
tasks: [
    {
        "type": "report",
        "when": ...,
        "options": {
            "to": [
                "test@test.com"
            ]
        }
    },
],
...