Add offline model and "new" form/controller

app/views/contests/offline_new.html.slim

@@ -0,0 +1,25 @@
+= form_with model: @offline, url: "/public/#{@contest.id}/offline" do |form|
+  .row.mb-3
+    .col
+      .form-text.mb-1
+        = t("puzzles.image_select")
+      = form.file_field :start_image, accept: "image/*", class: "form-control"
+      .form-text.error-message style="display: none;" id="image-error-message"
+        = t("puzzles.form.file_too_big")
+      javascript:
+        function setMaxUploadSize() {
+          const el = document.querySelector('input[type="file"]');
+          el.onchange = function() {
+            if(this.files[0].size > 2 * 1024 * 1024) {
+              document.getElementById('image-error-message').style.display = 'block';
+              this.value = "";
+            } else {
+              document.getElementById('image-error-message').style.display = 'none';
+            }
+          };
+        }
+
+        setMaxUploadSize();
+  .row.mt-3
+    .col
+      = form.submit t("helpers.buttons.add"), class: "btn btn-primary"