Ability for users to create Organizations and add/remove others to orgs

[bluepeter]

Similar to how it works in Github: users should be able to create organizations, and invite/add/remove/role other users/email addresses to that organization.

This is a pretty common use-case for B2B apps, including, e.g., Github, Trello, 15Five, and so on. It'd be useful here, too. (Although it may be too use-case-specific for this project.)