test: refactors blockstore integration tests to run as unit tests.

Tests which @requires_blockstore (i.e. the Blockstore service) have
been made to run as a unit test using the installed Blockstore app, and
will be run by the platform CI.

The Blockstore service tests can still be run manually by setting
EDXAPP_RUN_BLOCKSTORE_TESTS=1

Related fixes:

* adds blockstore bundle storage settings
* let the studio devstack and test servers serve static files from
  the /media URL This allows the blockstore/content libraries API to
  serve blockstore assets in dev.
* Wrap ContentLibrary creation in an atomic transaction, so that if it
 fails, the related bundle can be deleted directly from the database
  during the exception handler. (Previously, we called a REST API which
  deleted it as part of a separate service.)

cms/urls.py

@@ -271,6 +271,8 @@ if settings.DEBUG:
     except ImportError:
         pass
 
+    urlpatterns += static(settings.MEDIA_URL, document_root=settings.MEDIA_ROOT)
+
     urlpatterns += static(
         settings.VIDEO_IMAGE_SETTINGS['STORAGE_KWARGS']['base_url'],
         document_root=settings.VIDEO_IMAGE_SETTINGS['STORAGE_KWARGS']['location']