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/util/request.py

@@ -3,6 +3,7 @@ import re
 
 from django.conf import settings
 from microsite_configuration import microsite
+from xmodule.modulestore.locations import SlashSeparatedCourseKey
 
 COURSE_REGEX = re.compile(r'^.*?/courses/(?P<course_id>[^/]+/[^/]+/[^/]+)')
 
@@ -26,11 +27,17 @@ def course_id_from_url(url):
     """
     Extracts the course_id from the given `url`.
     """
-    url = url or ''
+    if not url:
+        return None
 
     match = COURSE_REGEX.match(url)
-    course_id = ''
-    if match:
-        course_id = match.group('course_id') or ''
 
-    return course_id
+    if match is None:
+        return None
+
+    course_id = match.group('course_id')
+
+    if course_id is None:
+        return None
+
+    return SlashSeparatedCourseKey.from_deprecated_string(course_id)