Make course ids and usage ids opaque to LMS and Studio [partial commit]

This commit updates common/djangoapps.

These keys are now objects with a limited interface, and the particular
internal representation is managed by the data storage layer (the
modulestore).

For the LMS, there should be no outward-facing changes to the system.
The keys are, for now, a change to internal representation only. For
Studio, the new serialized form of the keys is used in urls, to allow
for further migration in the future.

Co-Author: Andy Armstrong <andya@edx.org>
Co-Author: Christina Roberts <christina@edx.org>
Co-Author: David Baumgold <db@edx.org>
Co-Author: Diana Huang <dkh@edx.org>
Co-Author: Don Mitchell <dmitchell@edx.org>
Co-Author: Julia Hansbrough <julia@edx.org>
Co-Author: Nimisha Asthagiri <nasthagiri@edx.org>
Co-Author: Sarina Canelake <sarina@edx.org>

[LMS-2370]

common/djangoapps/track/contexts.py

@@ -1,7 +1,8 @@
 """Generates common contexts"""
 import logging
 
-from xmodule.course_module import CourseDescriptor
+from xmodule.modulestore.locations import SlashSeparatedCourseKey
+from opaque_keys import InvalidKeyError
 from util.request import COURSE_REGEX
 
 log = logging.getLogger(__name__)
@@ -9,15 +10,24 @@ log = logging.getLogger(__name__)
 
 def course_context_from_url(url):
     """
-    Extracts the course_id from the given `url` and passes it on to
+    Extracts the course_context from the given `url` and passes it on to
     `course_context_from_course_id()`.
     """
     url = url or ''
 
     match = COURSE_REGEX.match(url)
-    course_id = ''
+    course_id = None
     if match:
-        course_id = match.group('course_id') or ''
+        course_id_string = match.group('course_id')
+        try:
+            course_id = SlashSeparatedCourseKey.from_deprecated_string(course_id_string)
+        except InvalidKeyError:
+            log.warning(
+                'unable to parse course_id "{course_id}"'.format(
+                    course_id=course_id_string
+                ),
+                exc_info=True
+            )
 
     return course_context_from_course_id(course_id)
 
@@ -34,23 +44,12 @@ def course_context_from_course_id(course_id):
         }
 
     """
+    if course_id is None:
+        return {'course_id': '', 'org_id': ''}
 
-    course_id = course_id or ''
-    context = {
-        'course_id': course_id,
-        'org_id': ''
+    # TODO: Make this accept any CourseKey, and serialize it using .to_string
+    assert(isinstance(course_id, SlashSeparatedCourseKey))
+    return {
+        'course_id': course_id.to_deprecated_string(),
+        'org_id': course_id.org,
     }
-
-    if course_id:
-        try:
-            location = CourseDescriptor.id_to_location(course_id)
-            context['org_id'] = location.org
-        except ValueError:
-            log.warning(
-                'Unable to parse course_id "{course_id}"'.format(
-                    course_id=course_id
-                ),
-                exc_info=True
-            )
-
-    return context