Restricts triggering of alerts to messages tagged to topics

Alerts should not be triggered for messages regarding major changes
to documents that are not tagged to topics.

Story: https://www.pivotaltracker.com/story/show/79009772

Authored by @dp6ai and @pedrosmmoreira
After discussion we're going to make the topic check a responsibility of the `MessageProcessor`; it's a valid document without them, we're just deciding not to send the email.