[IMP] fields: make related fields readonly by default

With this commit, any *explicitly* related fields will be `readonly=True`
by default.

*Implicitly* related fields (i.e. _inherits fields) however will keep the
source field's `readonly` attribute.

The rationales behind this patch are:
    * Enforce good practices, as the most common use-case for
      related fields is the same as for compute fields: to read data.
    * Avoid errors where some user saves a form view which contains
      a related field that the user doesn't have write access to.

odoo/fields.py

@@ -421,6 +421,7 @@ class Field(MetaField('DummyField', (object,), {})):
             # by default, related fields are not stored and not copied
             attrs['store'] = attrs.get('store', False)
             attrs['copy'] = attrs.get('copy', False)
+            attrs['readonly'] = attrs.get('readonly', True)
         if attrs.get('company_dependent'):
             # by default, company-dependent fields are not stored and not copied
             attrs['store'] = False
@@ -621,7 +622,6 @@ class Field(MetaField('DummyField', (object,), {})):
     _related_comodel_name = property(attrgetter('comodel_name'))
     _related_string = property(attrgetter('string'))
     _related_help = property(attrgetter('help'))
-    _related_readonly = property(attrgetter('readonly'))
     _related_groups = property(attrgetter('groups'))
     _related_group_operator = property(attrgetter('group_operator'))
 

odoo/models.py

@@ -2480,6 +2480,7 @@ class BaseModel(MetaModel('DummyModel', (object,), {'_register': False})):
                     related=(parent_field, name),
                     related_sudo=False,
                     copy=field.copy,
+                    readonly=field.readonly,
                 )
 
         # add inherited fields that are not redefined locally