Add oe_read_only as inverse of class oe_edit_only

See https://www.odoo.com/forum/help-1/question/inverse-of-css-class-oe-edit-only-11853

Already present in branch 8.0, see https://github.com/odoo/odoo/blob/8.0/addons/web/static/src/css/base.css#L1833