Allow custom css in surveys

Right now, the design of a survey is limited by the few options given in the design template editor. More advanced styling is only possible when adding custom JavaScript to the survey, then inserting a stylesheet into the DOM from there. This is cumbersome and not very performant, i. e. there is a noticeable delay before these styles are applied to the page.

Please consider allowing to add custom CSS rules in the design editor or on a per survey basis.