Make caching in get_common_page_data optional

@sefk said he could do it
* Adds use_cache parameter (defaults to true) to get_common_page_data.
* Uses use_cache=False from inside manage_nav_menu to force a cache miss
  and guarantee admin sees recent changes. Incidentally also means that
  student view of course_info_pages gets pre-cached on the app node the
  admin happens to be working with.
  (closes #2098)