[#559] Implement APK download as Dashboard url

* Visiting the http://instance.akvoflow.org/app redirects the browser to
  the S3 location where the APK is stored
* The redirection is to the latest published APK for that instance
* If the instance doesn't have any published APK, a "NOT FOUND" (HTTP
  404) is replied
Merge pull request #560 from akvo/issue/559

[#559] Implement APK download as Dashboard url