Add expire mechanism for preview images about server

[rdlab-upc]

@alx-tuilmenau I totally agree with you. IMHO, there is a possible optimization for your proposal, instead of deleting all previews "pass due" at once, delete groups of 50/100/300 images every cron execution in order to decrease filesystem/DB stress.

Eventually, DB+filesystem will be "clean" without collateral issues. Most NC instances will have hundreds of users and thousands of previews...

[alx-tuilmenau]

@rdlab-upc of course, and this should be done during the maintenance time set in config.
I discovered it first because I had to wait a long time for re-organizing the underlying filesystem (for other reasons) and noticed it needed days for the preview folder. My ugly cleanup script needed about half a day to delete ca. 300k images and 400k folders older than 1 year (but the storage is on Nearline HDDs, so it's not fast).