As a user I want to be able to unsubscribe from task notifications in PRs, without unsubsrcibing from comment or push notifications

Problem Definition

Some users want to receive notifications for comments/new commits in the PRs they are watching, but they want to be able to unsubscribe from task notifications.

Suggested Solution

Allow users to subscribe/unsubscribe to/from specific events of a PR, e.g. tasks, comments, new commits, so they can select which events to subscribe to.