[WebHooks.Receivers.VSTS] Add Missing Release Event Handlers

[RyanMathewson]

The following events are configurable in VSTS but not handled by the VSTS receiver:

• Release created
• Release abandoned
• Release deployment approval completed
• Release deployment approval pending
• Release deployment completed
• Release deployment started

Are these being added?

Reference: VSTS Service Hooks Events

[dougbu]

We've moved this issue is in the Backlog milestone. This means that it is not going to happen for the coming release. We will reassess the backlog following the current release and consider this item at that time. However, keep in mind that there are many other high priority features with which it will be competing for resources.

Have also marked this issue as up-for-grabs. This means we will review a PR filling this gap if someone submits one.