[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.