Add JQuery timepicker CSS to the unit and container templates

Studio loads [JQuery timepicker](http://jonthornton.github.io/jquery-timepicker/) JS on the unit page, but not the CSS stylesheet.  I'm planning on using the timepicker in ORA2's XBlock editing view, so I need the stylesheet to be available.

This is not ideal, since ORA2 needs to make additional assumptions about what dependencies are available in Studio.  @cpennington This is a case where it would be really helpful for the XBlock runtime to manage client-side dependencies.

@andy-armstrong and @frrrances please review.