Add notifications for task events

# Problem

We should generate notifications for users on tasks for the following events:

- task description changed
- PR was opened for the task
- comment was added
- comment was edited

These notifications may need us to first have the notion of an event model. [See how GitHub does this](https://developer.github.com/v3/issues/events/).

We may also need to distinguish _watching_ notifications from _participating_ notifications. [See again how GitHub handles this](https://help.github.com/articles/about-notifications/), and we should look into, e.g., how Trello or Asana handle these.