course/index: minor context-related fixes

course/index: minor context-related fixes

 - The code uses the system context a lot. Declare
   $sysctx at the top and use it.

 - If the category has a context property, use it
   (saves 1 DB query per category displayed)

The DB costs of this page in editing mode are

 - ~100 DB queries for fix_course_sortorder()

 - ~50 DB queries for the categories listing

If we rewrite both to lose the recursion, we could resolve the
page in perhaps 5 DBq.