Add the ability to merge HTML reports

We want to be able to generate HTML reports for Staging and for any other project/environment where the QA tests are run.

The reporting for package-and-qa and review-qa-all uses a script, bin/merge_html_reports, which was added to the gitlab-qa and gitlab repositories. We should add that functionality to gitlab-qa so that the script doesn't have to be copied everywhere or extracted from the gem.