Refactor GitHub webhook processing logic into handlers

# Problem

> There are "classes" of webhooks, some of which pertain to the repository and project and other that don't (installation, e.g.). As CC adds more webhook handlers for various reasons, the `should_process?` function will become increasingly further away from the handler logic, so this should eventually move down into the event handler logic, with the controller merely being a dispatcher.

https://github.com/code-corps/code-corps-api/pull/1254#issuecomment-347049874