Add subscribe feedback and clarify {id} placeholder usage in link template

app/assets/javascripts/new_channel.js

@@ -311,16 +311,21 @@
     subscribe_button.unbind("click");
     subscribe_button.on("click",function(){
       dom.addClass("active");
+      subscribe_button.attr("disabled","disabled");
       $.ajax({
         url : "/admin/feeds/subscribe",
         data : {"url" : site_url, "feed" : feed, "channel" : channel.title, "channel_key" : channel.key, "category" : select.val()},
         dataType : "json",
         type : "post"
       }).done(function(){
+        subscribe_button.removeAttr("disabled");
         $("#selectCategoryModal").modal("hide");
+        alert("訂閱成功！");
+      }).fail(function(){
+        subscribe_button.removeAttr("disabled");
+        alert("訂閱失敗，請確認欄位設定是否正確，或稍後再試一次");
       })
     })
-
   }
 
   var displayLoading = function(display, msg){

app/views/admin/feeds/new.html.erb

@@ -74,6 +74,7 @@
           </div>
           <div id="json-link-template-wrapper" style="display:none;">
             連結樣板：<input type="text" id="json-link-template" class="manual-rss-input" placeholder="例如 https://news.pts.org.tw/article/{id}"><br/>
+            <small style="color:#888;">注意：一定要用固定文字 {id}，不要換成 ID 欄位實際的名稱（例如不要寫成 {news_id}）</small><br/>
           </div>
         </div>
       </div>