Block downgrading sender key support

Disallow linking an additional device to an account that has already
upgraded to having sender key support where the linked device does not
have sender key support. This should prompt the person attempting to
link the older application to upgrade in order to complete the linking
process.

service/src/main/java/org/whispersystems/textsecuregcm/controllers/DeviceController.java

@@ -234,6 +234,10 @@ public class DeviceController {
   private boolean isCapabilityDowngrade(Account account, DeviceCapabilities capabilities, String userAgent) {
     boolean isDowngrade = false;
 
+    if (account.isSenderKeySupported() && !capabilities.isSenderKey()) {
+      isDowngrade = true;
+    }
+
     if (account.isGv1MigrationSupported() && !capabilities.isGv1Migration()) {
       isDowngrade = true;
     }