Make report history accessible

[wowasa]

We're already creating reports with a timestamp in XML and HTML, which are stored in the file system but which are not available yet for download.
In an enhancement we want

1. to assure, that old reports accessible/downloadable via URL
2. have a link in each HTML report to the previous report
3. mark old reports in a way, that they can't be confused with the current report

[wowasa]

unfortunately we mustn't have links to link checker details and profile in the history files, since this would link to current reports (unless we decide to save a history for these reports, too).
Hence we can't just copy the current file by adding a timestamp to the file name anymore, as we're doing now, but we have to generate the HTML view of history reports with their own stylesheet

[wowasa]

done