Make HTML reports use external CSS style sheets

Current Situation

HTML reports generated by the Insight Collector use a built-in CSS definition. It is possible to override built-in styles using spec.style property of a SummaryReport insight, but it could be interesting to provide a possibility for external style sheets use.

Desired Outcome

Following the logic described in #622 (closed), make HTML reports use external CSS style sheets that can be found in the configurations_path directory css subfolder.

If the configurations_path/css directory contains more than one CSS file, files content should be concatenated following files priority.