Handle teams

[21echoes]

References to teams (e.g., assigning a team as a reviewer) should use the GitHub API to convert that team into a list of individuals, and then we should perform the usual behavior (e.g., DM notification) for each of them. Notifications failing for one team member should not short-circuit and prevent other notifications from going out.

By default, the token we are given (the secrets.GITHUB_TOKEN env var) does not seem to have the requisite permissions to fetch teammates: https://docs.github.com/en/free-pro-team@latest/actions/reference/authentication-in-a-workflow#permissions-for-the-github_token so we'll either need to:

• find a different way to get the list of users directly from the existing payload? it's a huge payload, maybe it's hidden in there somewhere 🕵️
• every repo which wants to use this action will need to provide their own custom token with the requisite permisisons 😢