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/static_replace/__init__.py

@@ -72,7 +72,7 @@ def replace_jump_to_id_urls(text, course_id, jump_to_id_base_url):
     return re.sub(_url_replace_regex('/jump_to_id/'), replace_jump_to_id_url, text)
 
 
-def replace_course_urls(text, course_id):
+def replace_course_urls(text, course_key):
     """
     Replace /course/$stuff urls with /courses/$course_id/$stuff urls
 
@@ -82,6 +82,8 @@ def replace_course_urls(text, course_id):
     returns: text with the links replaced
     """
 
+    course_id = course_key.to_deprecated_string()
+
     def replace_course_url(match):
         quote = match.group('quote')
         rest = match.group('rest')