Fix authorizing PL changes when users_default≠0 (#339)
Fix authorizing PL changes when users_default≠0 (#339)

`Users` map access would return 0 for users with no explicit power
level, even though it should default to `users_default`. The existing
`UserLevel()` helper method already handles it correctly, so this simply
switches power level event auth to using that method.

Signed-off-by: Tulir Asokan <d52a095ed9643bd427acb6654f10f4a52fda6a79@maunium.net>
matrix-org/gomatrixserverlib,matrix-org/gomatrixserverlib