Initial room power-levels are overly special-cased

Currently the [auth rules](https://matrix.org/docs/spec/server_server/unstable.html#rules) have ~~three~~ two special-cases for power levels before the first power_levels event lands:

1. the first power-level event is allowed regardless of its content
2. the creator of the room has PL 100
3. ~~state_default is 0 rather than 50.~~ Fixed by https://github.com/matrix-org/matrix-spec-proposals/pull/1656

I think we only need one of those.

Read the Matrix auth rules linked in the issue and trace the three initial power-level special cases, noting that the state_default case was already addressed by MSC1656. Decide which remaining exception should be retained, then update the specification so the initial power-level behavior has a single consistent rule.

Written by the indexing model from the issue text.