[FIX] web_dialog_size: Fix usage for non-admins and reduce calls (#954)

This addon had 2 problems after migrating to v11:

1. One call to backend's `ir.config_parameter.get_param` was done for
   each instantiated dialog, while the setting served for the whole
   session equally.
2. That model is now readable only by admin users, so non-admins
   couldn't use the default at all.

Fixed now.

web_dialog_size/models/__init__.py

@@ -0,0 +1 @@
+from . import ir_config_parameter