Truncate or zero out log files

[gbnewby]

Logfiles in cache/epub and ~gutenbackend/ebookconverter and perhaps elsewhere have grown quite large.

It should be safe to zero out the logfiles in cache/epub, and perhaps we should set a size limit of 1MB or less so old records are cut with every rebuild cycle.

For the ebookconverter logs, those could be mv'd and then new logfiles started. Should we set up a logrotate process to manage this? I'm not sure how long we should retain those logs for - perhaps 1 year, but we could do a monthly rotation.