Provide infrastructure for live documents

Resint should provide infrastructure for live documents. Live documents are media files that are generated at periodic intervals, and stored in their most recent version. This infrastructure can be used by features like the substitution plan generator in Chronos, that publishes an updated version of a print-out substitution plan PDF on plan changes.

The media files should be provided under an OAuth scope to applications that have undergone Client Credentials authentication (attention: currently, django-oauth-toolkit does not support limiting available scopes per application, cf. https://github.com/jazzband/django-oauth-toolkit/issues/262, so all OAuth applications will be able to retrieve these media files, for now - maybe this is only a limitation of the Authorization Code Flow; I will investigate).