Simplify authorization model

[mattias-p]

• Remove the add_api_user method.
• Add an optional configuration property for specifying a single authorization token.
• Remove the username parameter from the add_batch_job method.
• Remove the Username datatype from the RPCAPI Reference documentation.
• Remove the user store from the database schema.

[matsduf]

This update will involve both zonemaster/zonemaster and zonemaster/backend repositories.

[matsduf]

Most updates in specifications and documentations for this issues are found in zonemaster/zonemaster#1406.