Big access control refactor

* All access control logic is now in access.py
* It exports a single method for general use: has_access(user, object, action)
 - possible actions depend on object type (e.g. 'see_exists', 'enroll', 'staff')
* Removed DARK_LAUNCH feature flag--it is now the default behavior
* Replaced check_course with three separate more focused functions that use has_access

Minor things:
* note on using pdb in testing
* moved time parsing helper into timeparse.py
* x_modules now have a .start attribute (None if not in metadata)

lms/templates/course_navigation.html

@@ -7,7 +7,7 @@ def url_class(url):
   return ""
 %>
 <%! from django.core.urlresolvers import reverse %>
-<%! from courseware.courses import has_staff_access_to_course_id %>
+<%! from courseware.access import has_access %>
 
 <nav class="${active_page} course-material">
   <div class="inner-wrapper">
@@ -28,7 +28,7 @@ def url_class(url):
 % if user.is_authenticated():
       <li class="profile"><a href="${reverse('profile', args=[course.id])}" class="${url_class('profile')}">Profile</a></li>
 % endif
-% if has_staff_access_to_course_id(user, course.id):
+% if has_access(user, course, 'staff'):
       <li class="instructor"><a href="${reverse('instructor_dashboard', args=[course.id])}" class="${url_class('instructor')}">Instructor</a></li>
 % endif