Webhooks/events for integration with external services

[knrafto]

We should be able to configure (on a per-course or per-assignment basis) actions that will be triggered when certain events happen, such as receiving a new backup. This could either send an HTTP POST request to a URL, or call one of our "extensions" that can be used for this purpose. This will allow cleaner separation of concerns, and allow staff to swap in their own custom autograder instead of using ours.

Potential use cases:

• Running the autograder for each new submission
• Autograding all submissions
• Running moss on all submissions
• Uploading to Canvas API on each new score
• Notifying an online cheating detection system with each new backup
• Updating hog contest rankings for each new submission

We should look at the GitHub webhooks docs for inspiration.

[Sumukh]

Here's an implementation:

• There are a certain set of events that we support (New Backup, New Submission, New Score, New Group)
• There'll be a webhook table that will allow us to create webhooks specific to a course or specific to an assignment.
• The route responsible for creating the new object will also be responsible for sending the webhook.
  • The better, but more annoying alternative is to have commit hooks on the SQLAlchemy models
• The actual sending of the webhook will be a deferred task (with its own queue).
  • It should be delayed because sending the webhooks should not ever delay the original task.

[knrafto]

• +1 on sending webhooks being a deferred task. We should also retry delivery, and show which delivery attempts have failed.
• The body of the webhook request should contain the JSON representation of the new object.

Here's a possible schema:

class Webhook(Model):
    event_type  # enum of possible event types
    course_id
    assignment_id  # nullable. if null, applies to entire course.
    webhook_url  # URL to POST to
    secret_token  # security token to include with webhook request

I'm not sure how this would handle our own integrations (e.g. bCourses). We could just fill in an internal URL (e.g. the URL is https://okpy.org/...)

[Sumukh]

A course has requested a basic version of this (trigger a webhook upon submission).