Use Danger to automatically apply labels

For Gitaly's merge requests @GitalyBot will come by and apply labels based on the merge request, and its changelog entry. Which allows fewer MRs to go unlabeled and improve tracking for throughput, but also allows team members to create a changelog entry and worry about less process.

Gitaly's approach works less well for a bigger project like gitlab-ce, mostly as applying the team label checks if the user name is any or a static set. Now this could be checked against @gl-gitaly, allowing for a more salable and dynamic approach.

/cc @DouweM