Improve the reproducible build process.

* Moved stuff into it's own `reproducible-builds` directory.
* Improved reproducible build by using a debian snapshot and more clearly listing dependencies.
* Removed signing from assembleReelase.
* Updated README.

app/build.gradle

@@ -454,16 +454,6 @@ task signProductionWebsiteRelease {
     }
 }
 
-tasks.whenTaskAdded { task ->
-    if (task.name.equals("assemblePlayRelease")) {
-        task.finalizedBy signProductionPlayRelease
-    }
-
-    if (task.name.equals("assembleWebsiteRelease")) {
-        task.finalizedBy signProductionWebsiteRelease
-    }
-}
-
 def getLastCommitTimestamp() {
     new ByteArrayOutputStream().withStream { os ->
         def result = exec {