Add contestant export feature

app/views/contestants/export.csv.slim

@@ -0,0 +1,4 @@
+= CSV.generate_line [t("helpers.rank"), t("activerecord.attributes.contestant.name"), t("activerecord.attributes.contestant.display_time"), t("activerecord.attributes.contestant.completions")]
+
+- @contestants.each_with_index do |contestant, index|
+  = CSV.generate_line([index + 1, contestant.name, contestant.display_time, contestant.completions.length])

app/views/contests/show.html.slim

@@ -40,8 +40,10 @@ javascript:
           = t("contestants.plural").capitalize
           a.ms-3.btn.btn-sm.btn-primary href=new_contest_contestant_path(@contest) style="margin-top: -3px"
             | + #{t("helpers.buttons.add")}
-          a.ms-2.btn-sm.btn.btn-primary href=contest_import_path(@contest) style="margin-top: -3px"
+          a.ms-2.btn.btn-sm.btn.btn-primary href=contest_import_path(@contest) style="margin-top: -3px"
             | #{t("helpers.buttons.import")}
+          a.ms-2.btn.btn-sm.btn.btn-primary href="/contests/#{@contest.id}/export.csv" style="margin-top: -3px"
+            | #{t("helpers.buttons.export")}
     - if @contest.categories.size > 0
       .row
         .col