Fixes #5983; possibly also has a bearing on #4306. A CSS rule dating from the original StreamField design was hiding the 'required' asterisk on fields within a required StreamField, presumably on the mistaken belief that they duplicate the information given by the top-level asterisk on the streamfield. (In #5983 this was reported as a regression in 2.7 when the react-streamfield CSS was introduced, so it's possible that the old design had something to mitigate this, e.g. an asterisk being inserted elsewhere.)