Fix url field mobile width

jet/static/jet/css/_changeform.scss

@@ -449,7 +449,11 @@ body.popup .submit-row {
 }
 
 .vURLField {
-  width: 30em;
+  width: 26em;
+
+  @include for-phone {
+    width: 100%;
+  }
 }
 
 .vLargeTextField, .vXMLLargeTextField {