Users of a Mantissa server can vary wildly from one to the next. Some users may want email, some may want games, many may want some unique combination of the two. Mantissa administrators need to configure two different things:

• What features are available to newly-created user accounts? As a sub-configuration of this, there may be different kinds of user accounts for which new users can sign up.
• What features are available as add-ons for existing user accounts?

These choices are bounded by actions taken by the system administrator: what Mantissa plugins have been installed on the system running the Mantissa server? Once installed, these plugins are found by the Mantissa server and made available to the Mantissa administrator via some interface (most likely web, initially). From this list, the Mantissa administrator can configure kinds of user accounts and select which features can be added to existing accounts.

[TODO: Explain how this is reflected in the database]