Handle Installation::deleted event

# Problem

A user can delete a github installation from github. In that case, pretty much all of github integration will stop working.

We should handle the `Installation::deleted` event to avoid potential issues.

- receive event
- find and delete associated `GithubAppInstallation` (alternative: set state to "uninstalled")
- find and disassociate associated `Project`
- find and delete associated `GithubRepo` records (alternative: add state field, set state to "uninstalled").
- find and delete associated `ProjectGithubRepo` records

Handling this one is still unclear to me and warrants additional discussion.

https://developer.github.com/v3/activity/events/types/#installationevent