Unify bots used in the repo

[m-allanson]

Summary

There are several bots used to automate different parts of the repo. Unify them into one single bot - gatsbot - which is managed by Gatsby's Peril setup.

Basic example

Existing bots are:

• stale bot - currently disabled?
• gatsbybot - comments when deploy previews are ready
• gatsbot - this is the new Peril-backed bot for inviting new contributors to the gatsbyjs org
• @KyleAMathews's account - hooked up to post a comment when deploy previews fail (I don't think he's posting all those links himself 🤖) - example

Motivation

Centralise repo automation in one place to make it easier to manage. Provide a more consistent experience to contributors.

[LekoArts]

@m-allanson This issue is resolved, right?

[m-allanson]

It is!

Edit: gatsbot is now in charge of automation around here, the other bots have retired (including robot Kyle).