Make publicly hosted GitHub app

[brandon-leapyear]

It would be a "nice-to-have" to have a publicly available Mergit GitHub app that people can use to quickly install Mergit on their repos, without needing to deploy their own instance.

Currently, Mergit isn't set up for multi-tenant access:

• Non-LY accounts going to Mergit web UI will attempt to load private LY repos (Secure Web UI for public access #201)
• Anyone with access to a repo will be able to reset merge runs (Secure Web UI for public access #201)
• We currently have a global queue which delegates events into per-repo queues, so if we have too much traffic, the global queue will get filled up (alternatively, have a private LY Mergit instance + a public one, and remove the deploy scripts in favor of instructions to deploy one's own Mergit instance)