Replace also_allow_user with a global config option

Basically reverts 088977f676.

This way is more suitable for self-hosting where there's no gateway to
manage the query parameter.

synapse/config/_base.pyi

@@ -17,6 +17,7 @@ from synapse.config import (
     jwt,
     key,
     logger,
+    meow,
     metrics,
     modules,
     oidc,
@@ -65,6 +66,7 @@ class RootConfig:
     voip: voip.VoipConfig
     registration: registration.RegistrationConfig
     account_validity: account_validity.AccountValidityConfig
+    meow: meow.MeowConfig
     metrics: metrics.MetricsConfig
     api: api.ApiConfig
     appservice: appservice.AppServiceConfig