Use portal_catalog instead of uid_catalog to avoid Unauthorized getObject

The plonebooking_add script raised the following error when you are coming from the plonebooking_add_form
```
Module script, line 23, in plonebooking_add
    <FSPythonScript at /plonebooking_add used for />
    Line 23
Unauthorized: You are not allowed to access 'getObject' in this context
```
The reference_catalog and uid_catalog getObject methods are no more callable in Python (Script) and Page Templates.

We can use the portal_catalog to get the object instead.