Skep: Slack plugin

A Slack implementation for Skep.

Features

This plugin will create the following for each team in the org:

1. A Slack user group for the team, so it can be called out in Slack with @team-name (paid version only).
2. A Slack user group for each sub-team, e.g. devs-only, etc (paid version only). Sub-teams are defined at the org level as a map of sub-team-name => function that filters members of the team based on their Person properties.
3. A private Slack channel for the team (paid version only). The Slack user group is used to define membership in this channel. This can optionally be turned off. The standard prefix for the channel name is defined at the org level.
4. A public Slack channel for the team. The Slack user group is used to define membership in this channel (paid version), or team members will need to join manually (free version). This can optionally be turned off.