Resolve #3244695 "User email constraint"

Closes #3244695

password_policy_mail/config/schema/password_policy_mail.schema.yml

+# Schema for configuration files of the Password Policy Username submodule.
+
+password_policy.constraint.plugin.password_mail:
+  type: password_policy.constraint.plugin
+  mapping:
+    disallow_mail:
+      type: boolean
+      label: 'Disallow mail'