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.

Interestingly, there is some disagreement as to whether these links should be in ```header_extras``` or ```jsextra```. Of the five occurrences in the code, one uses ```header_extras```` and the other four use ```jsextra```.

I would have thought that ```header_extras``` was the more natural place to put CSS, but I think that ```jsextra``` also ends up in the header so it shouldn't matter.

So, this is fine...