✨(documents) allow retrieving versions (list and detail)

Versions are retrieved directly from object storage and served on API
endpoints. We make sure a user who is given access to a document will
only see versions that were created after s.he gained access.

src/backend/impress/settings.py

@@ -137,6 +137,8 @@ class Base(Configuration):
         environ_prefix=None,
     )
 
+    S3_VERSIONS_PAGE_SIZE = 50
+
     # Internationalization
     # https://docs.djangoproject.com/en/3.1/topics/i18n/