Org installations in Node.js

[colmdoyle]

See #7 for more context.

[colmdoyle]

Looks like we have various callsites where body.team.id is assumed to be present and consistent, so will need to rip and replace those.

[colmdoyle]

Will also need to refactor places where we use User.findOrCreate(), like https://github.com/slackapi/tasks-app/blob/main/nodejs/utilities/reload-app-home.js