If no cursor is specified and negative num_skip is used, we're pointing
one record back from the last one, so host logs always returned 101
lines as the default. This was also the case of the lines query argument
that used the number directly as num_skip. Instead of doing that, point
N-1 records to the back and then get N records. Handle 1 record and
invalid numbers silently to avoid the need for error handling in
unpractical edge cases.
* 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
