Add get_or_create_from_event method to Installation manager #100
Description
Right now, when the installation_repositories event comes in we just get an existing Installation model from the db and if it doesn't exist, nothing happens.
This can lead to slightly confusing onboarding for the library (see #95) -- we have a management command for importing an existing GitHub app and docs explaining that's the right way to use the library with an existing one, however it would be a bit easier and more user-friendly IMO if there was a get_or_create_from_event method so when that event comes in if the Installation doesn't exist in the db we can create one from the payload received (we have all the information we need from it).