Event stream per label

To keep track of work being done by certain teams it would be nice to have an event stream of sorts per label. For example, for the gitlab-ce27312481 label I'd like to see what issues/merge requests are created/closed/merged, and maybe any comments left on the Issuables with the gitlab-ce27312481 label. As a start we can exclude comments. Using such an event stream it becomes trivial to see what team members are working on.

cc @DouweM @JobV