[IMP] base, *: tighten `ir.model` access rights

Remove unrestricted "read" access.  To make code internally using `ir.model`
work, add a private method `_get` on `ir.model` to retrieve the record
corresponding to a model name, without access rights issue.

Change signature of method `get_authorized_fields` to make it use a model name
instead of a model id.  This removes the necessity of a search on `ir.model`.