Handle Installation::deleted event

[begedin]

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

[joshsmith]

We should discuss this but I think punt on it for now, since we'll be the only installation for a little while.