Prevent browsers from caching updated (minified) CSS/JS files in non-debug mode

Problem:
On a production website, end users often need to refresh a page or clear local browser cache to see the last changes the developer has made to a stylesheet or javascript file.

Cause:
The time-stamping feature is never used in non-debug mode.

Solution:
Enable time-stamping feature by default (when parameter is not given and when in non-debug mode).
The timestamp used is the modified timestamp of the original file (not the timestamp from parameters.yml)

Note:
This also works for non-minified files.