[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

web_dialog_size/models/ir_config_parameter.py

@@ -0,0 +1,17 @@
+# Copyright 2018 Tecnativa - Jairo Llopis
+# License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl).
+
+from odoo.models import api, Model
+from odoo.tools.safe_eval import const_eval
+
+
+class IrConfigParameter(Model):
+    _inherit = "ir.config_parameter"
+
+    @api.model
+    def get_web_dialog_size_config(self):
+        get_param = self.sudo().get_param
+        return {
+            key: const_eval(get_param("web_dialog_size.%s" % key, "False"))
+            for key in ["default_maximize"]
+        }