year specification in the copyright header

We should define the further procedure for the year specification in the copyright header. The discussion started already in the commit 67679892: "Update copyrightHeader year".

The latest proposal is, that there are two years mentioned with comma separation: "The {year} is the year in which the content was created (e.g. “2004”). The date may optionally be set to a range of years with the first and last years of the range separated by a comma (e.g. “2004, 2017”); in this case, the first year is when the content was first created and the last year is when the content was last modified."

@all Please comment