Refactor github webhook to handle multiple payload verions
split github fixtures into v1 and v2 versions
Group together all realm-specific logic. When v2 becomes available, we can
ask someone in each org to make the changes via the Github Hook configuration, and
slowly remove the special cases.
TODO: when our pull request for github-services gets merged, the integrations page
should say to look for Zulip instead of Humbug
(imported from commit 4790a730010b37186640f9996291afa6e8f96c2b)
acrefoot