Replace custom webhooks via githubapi integration

[Gormartsen]

Comments to pull request.

• Payload is object now, not an array.
• I did not rename project_github.pages.inc to project_github.payload.inc to keep diff more visible.
• githubapi do not return error message from hook_githubapi_payload to Github Payload for now.

[quicksketch]

I squashed the commits into a single one, plus a followup for code style and an issue with the README not getting updated:

This PR: https://github.com/backdrop-contrib/project/commit/73ed764db4b64b0ca711ba864549bbf6f65658ca Follow up: https://github.com/backdrop-contrib/project/commit/9759c54b11c99d6cb2caff8cb837056547a7abb0

Thanks @Gormartsen! I always thought eventually we'd move to a dependency that was more centralized. This helps tremendously. Look at all that code that was duplicated! Thanks!