Require contest durations, prefill end times for unfinished puzzles & allow to modify them

app/views/contests/new.html.slim

@@ -4,6 +4,12 @@
       .form-floating
         = form.text_field :name, autocomplete: "off", class: "form-control"
         = form.label :name, class: "required"
+  .row.mb-3
+    .col
+      .form-floating
+        = form.text_field :duration, autocomplete: "off", class: "form-control"
+        = form.label :duration, class: "required"
+        .form-text = t("activerecord.attributes.contest.duration_description")
   .row.mt-4
     .col
       = form.submit t("helpers.buttons.create"), class: "btn btn-primary"