Save user state for Blockstore XBlocks in CSM, clean up CSM a bit (#21630)

This commit introduces the changes needed for XBlocks in Blockstore to save
their user state into CSM. Before this commit, all student state for Blockstore
blocks was ephemeral (in-process dict store).

Notes:

* The main risk factor of this PR is that it adds non-course keys to the
  course_id field in CSM. If any code (like analytics?) reads course keys
  directly out of CSM and doesn't have graceful handling for key types it
  doesn't recognize, it could cause an issue. With the included changes to
  opaque-keys, calling CourseKey.from_string(...) on these values will raise
  InvalidKeyError since they're not CourseKeys. (But calling
  LearningContextKey.from_string(...) will work for both course and library
  keys.)
* This commit introduces a slight regression for the Studio view of XBlocks in
  Blockstore content libraries: their state is now lost from request to request.
  I have a follow up PR to give them a proper studio-appropriate state store,
  but I want to review it separately so it doesn't hold up this PR and we can
  test this PR on its own.

common/lib/xmodule/xmodule/video_module/video_module.py

@@ -23,7 +23,6 @@ from operator import itemgetter
 import six
 from django.conf import settings
 from lxml import etree
-from opaque_keys.edx.keys import CourseKey
 from opaque_keys.edx.locator import AssetLocator
 from web_fragments.fragment import Fragment
 from xblock.completable import XBlockCompletionMode
@@ -201,7 +200,7 @@ class VideoBlock(
         return waffle_flags()[DEPRECATE_YOUTUBE].is_enabled(self.location.course_key)
 
     def youtube_disabled_for_course(self):
-        if not isinstance(self.location.course_key, CourseKey):
+        if not self.location.context_key.is_course:
             return False  # Only courses have this flag
         if CourseYoutubeBlockedFlag.feature_enabled(self.location.course_key):
             return True