Integrate postnumber resolver across MAS flows

templates/pages/upstream_oauth2/do_register.html

@@ -102,6 +102,7 @@ Please see LICENSE files in the repository root for full details.
       {% call(f) field.field(label=_("common.username"), name="username", form_state=form_state) %}
         <input {{ field.attributes(f) }} class="cpd-text-control" type="text" autocomplete="username" autocorrect="off" autocapitalize="none" value="{{ imported_localpart or '' }}" aria-describedby="{{ f.id }}-help" data-choose-username />
         {{ field.error(error={"kind": "policy", "code": "username-invalid-chars"}, hidden=true) }}
+        {{ field.error(error={"kind": "policy", "code": "postnumber-reserved"}, hidden=true) }}
 
         {% if f.errors is empty %}
           <div class="cpd-form-message cpd-form-help-message" id="{{ f.id }}-help">