[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.

(cherry picked from commit ab30590f)
Signed-off-by: Marcelo Vanzin <vanzin@cloudera.com>