[#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
[#565] Adds "Download FLOW app" footer link
[#565] Better translations

* Based on @loicsans suggestion (in French)
Merge pull request #567 from akvo/issue/565

[#565] Adds "Download FLOW app" footer link
[#568] RELEASE NOTES v1.6.17
[#568] updating release notes with minor corrections
Merge pull request #569 from akvo/issue/568

Release Notes v1.6.17
[#572] Redirect to download SatStat APK
[#572] Redirect to download SatStat APK

* Changed URI to /gps
* Using Datastore entities instead of Properties file to store redirect address
[#572] Remove unused import
[#572] Using neutral constant for GPS URI
Merge pull request #573 from akvo/issue/572

[#572] Add URI to download SatStat APK
[#568] Update Release Notes
Merge branch 'release/1.6.17' of github.com:akvo/akvo-flow into develop
Merge branch 'develop' into feature/point-updates-v4