* Use Systemd Journal API for all logs endpoints in API
Replace all logs endpoints using container logs with wrapped
advanced_logs function, adding possibility to get logs from previous
boots and following the logs. Supervisor logs are an excetion where
Docker logs are still used - in case an exception is raised while
accessing the Systemd logs, they're used as fallback - otherwise we
wouldn't have an easy way to see what went wrong.
* Refactor testing of advanced logs endpoints to a common method
* Send error while fetching Supervisor logs to Sentry; minor cleanup
* Properly handle errors and use consistent content type in logs endpoints
* Replace api_process_custom with reworked api_process_raw per @mdegat01 suggestion
