Add Slack option for onboard command

[thekaveman]

Acceptance criteria

• --slack or similar option should create a new user in Slack with a default set of channels
• Based on --account-type, add user to more/different channels
• Based on --account-type, add user to Slack group

Research

This may not be possible unless we upgrade to an Enterprise plan: https://api.slack.com/admins

The following list of features and APIs are available in Enterprise Grid workspaces only.