Logging

[gregzaal]

For some unpredictable processes like thumbnail generation, collect debugging logs in the data folder. Only keep logs up to a week old in a single cycling file.

Don't collect any personal info. The log file should not be uploaded anywhere automatically, it's just for the user to send to me upon request if they run into a bug.

[gregzaal]

Done in da856f6fc90a5 and 3e54814bb2