[SPARK-20517][UI] Fix broken history UI download link

The download link in history server UI is concatenated with:

```
 <td><a href="{{uiroot}}/api/v1/applications/{{id}}/{{num}}/logs" class="btn btn-info btn-mini">Download</a></td>
```

Here `num` field represents number of attempts, this is not equal to REST APIs. In the REST API, if attempt id is not existed the URL should be `api/v1/applications/<id>/logs`, otherwise the URL should be `api/v1/applications/<id>/<attemptId>/logs`. Using `<num>` to represent `<attemptId>` will lead to the issue of "no such app".

Manual verification.

CC ajbozarth can you please review this change, since you add this feature before? Thanks!

Author: jerryshao <sshao@hortonworks.com>

Closes #17795 from jerryshao/SPARK-20517.

core/src/main/resources/org/apache/spark/ui/static/historypage-template.html

@@ -77,7 +77,7 @@
       <td><span title="{{duration}}" class="durationClass">{{duration}}</span></td>
       <td>{{sparkUser}}</td>
       <td>{{lastUpdated}}</td>
-      <td><a href="{{uiroot}}/api/v1/applications/{{id}}/{{num}}/logs" class="btn btn-info btn-mini">Download</a></td>
+      <td><a href="{{log}}" class="btn btn-info btn-mini">Download</a></td>
       {{/attempts}}
     </tr>
   {{/applications}}

core/src/main/resources/org/apache/spark/ui/static/historypage.js

@@ -120,6 +120,9 @@ $(document).ready(function() {
           attempt["startTime"] = formatDate(attempt["startTime"]);
           attempt["endTime"] = formatDate(attempt["endTime"]);
           attempt["lastUpdated"] = formatDate(attempt["lastUpdated"]);
+          attempt["log"] = uiRoot + "/api/v1/applications/" + id + "/" +
+            (attempt.hasOwnProperty("attemptId") ? attempt["attemptId"] + "/" : "") + "logs";
+
           var app_clone = {"id" : id, "name" : name, "num" : num, "attempts" : [attempt]};
           array.push(app_clone);
         }