Implement offline POST method for starting an offline participation

app/views/contests/offline_new.html.slim

@@ -1,8 +1,13 @@
-= form_with model: @offline, url: "/public/#{@contest.id}/offline" do |form|
+= form_with model: @offline, url: "/public/#{@contest.friendly_id}/offline" do |form|
+  .row.mb-3
+    .col
+      .form-floating
+        = form.text_field :name, autocomplete: "off", class: "form-control"
+        = form.label :name, class: "required"
   .row.mb-3
     .col
       .form-text.mb-1
-        = t("puzzles.image_select")
+        = t("offlines.start_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")
@@ -20,6 +25,6 @@
         }
 
         setMaxUploadSize();
-  .row.mt-3
+  .row.mt-4
     .col
-      = form.submit t("helpers.buttons.add"), class: "btn btn-primary"
+      = form.submit t("helpers.buttons.start"), class: "btn btn-primary"