HTML report is missing the `skip` status

[comps]

https://github.com/teemtee/tmt/pull/2326 added skip as a valid status, but the HTML report code hasn't been updated to support it:

        #results td.pass {
            background: #0a0;
        }
        #results td.fail {
            background: #d30;
        }
        #results td.info {
            background: #58d;
        }
        #results td.warn {
            background: #fc5;
        }
        #results td.error {
            background: #b4d;
        }

and on other places (checkboxes, etc.).