Log imports in file as well as stdout

[Rikuoja]

Is your feature request related to a problem? Please describe.
Currently, running imports only print to stdout. When running imports non-interactively (e.g. starting an import process with Flask), we would like to have the import logs saved to file.

Describe the solution you'd like
Use logging module.

[Rikuoja]

Currently imports are logged per import process, so we can easily find out what was wrong with a particular dataset and when it crashed.

Probably we want to add a single log file stream to the same logger too, so we would have a complete server log stream available.