Organization-level Webhooks

Hi,
I've added support for organization webhooks. I took the path of least resistance here by using most of the repo-based webhook code and just making two organization-specific templates.

Also made a few bugfixes along the way (edits for webhooks weren't working) and corrected the grammar for one of the messages.

Closes #378.

Let me know any feedback and I can incorporate.

Thanks!

\-Chris