Optimize the webhooks to better handle events.

• The labeled or unlabeled events should only try to update labels on an issue
• The milestoned or unmilestoned events should only update the item's milestone
• The assigned and unassigned events should be ignored as unsupported (we aren't tracking assignees)
• For pull requests, the synchronize event can update the full record, but we should stop displaying this activity in the timeline (see https://developer.github.com/v3/activity/events/types/#pullrequestevent and GitHub's note about it)
• The opened, closed, reopened, and edited events behave as they do now
• We change the code to be a bit more defensive; unknown events shouldn't attempt to be processed by the hooks by default