Expand GitHub issue ticket functionality

[k-mahoney]

In working through implementing out first broadcast and self-service alerts, we've found that more passive error reporting in the associated GitHub issue would be ideal - rather than failing outright or escalating immediately to PD. For Slack errors specifically, @zmully has laid out a lot of the initial work on this in #78.

Similarly, it would make sense to have a dispatch route that simply ingested the incoming SNS and opened an issue ticket and tagged a specified team or team member. We currently have priority, self-service, and broadcast alert types - so maybe this could be low-priority.

[k-mahoney]

This is complete via https://github.com/mapbox/dispatch/pull/122.