Statistics export url should explicitly request CSV via query parameter

[hsilva-keep]

ATM, we register an event handler for statistics export:

element.click(function() {
        var type = 'text/csv';
        $.ajax({
          accepts: {
            text: type
          },
          url: url,
          processData: false,
          dataType: 'text',
          success: function(data) {
            saveAs(new Blob([data], {
              type: type
            }), filename);
          }
        });
      });

But, in a scenario where the browser also adds some accepts headers, in front of our, the response might not be CSV.

E.g.

curl -u admin:PWD 'https://demo.roda-community.org/api/v1/index?returnClass=org.roda.core.data.v2.risks.IndexedRisk&&facet=currentSeverityLevel&lang=pt_PT&onlyActive=false&exportFacets=true&filename=risk-severity.csv' -H 'Accept: application/xml, text/csv, */*; q=0.01'

where it should be

curl -u admin:PWD 'https://demo.roda-community.org/api/v1/index?returnClass=org.roda.core.data.v2.risks.IndexedRisk&&facet=currentSeverityLevel&lang=pt_PT&onlyActive=false&exportFacets=true&filename=risk-severity.csv&acceptFormat=csv' -H 'Accept: application/xml, text/csv, */*; q=0.01'

[nunovieira220]

Done on b9ad289