Cache asciidoc content in db (Part of #394)

- Add RenderedContent model and related helper methods
- Change StaticContentView `get()` logic to try the cache, then the db, then S3
- Change StaticContentView to update db appropriately
- Refactoring for readability/maintainability

conftest.py

@@ -7,6 +7,7 @@ from django.core.files import File as DjangoFile
 # Include the various pytest fixtures from all of our Django apps tests
 # directories
 pytest_plugins = [
+    "core.tests.fixtures",
     "libraries.tests.fixtures",
     "news.tests.fixtures",
     "users.tests.fixtures",