update: add GH webhook event check for "installation" type events

• update: add GH webhook event check for "installation" type events

Enabled by default, installation type events get triggered when the associated GH application is installed into an organization or repository. By extracting the current scheduled task to the GitHub helper, we can call this arbitrarily when there are new installations as well as handle catch-up on a regular schedule.

Resolves #140 (closed)