[entities] cw_rest_attr_info() should only consider required attributes (closes #3766717) This prevents CW from choosing unique but non-required attributes. None/NULL is a poor choice for RESTful URIs.

bgcolor = '#000000'
stylesheets = ['%s/cubicweb.css' % datadir_url]
logo = '%s/logo.png' % datadir_url
lazy = lazystr('%(bgcolor)s')