Specify the default power levels during creation better

[turt2live]

Link to problem area: https://spec.matrix.org/v1.15/client-server-api/#creation

Issue
Currently, the spec is light on details for what the initial power levels event should include. Aside from state_default and creator permissions, we should also specify which events are restricted, the defaults for ban, redact, etc, and any other semantic behaviour.

Without this, clients are essentially forced to always use power_level_content_override to achieve deterministic power levels. If the spec was clear on what the defaults are, most clients could then rely on /createRoom doing the "right" thing.