backups (tgz): save integrity information

we generate a signing key pair for each target. Initially, I had this
as global. We needed a route to return the public key and putting it
under backup target seemed natural. Since we delete the backups when
we delete a target, we lose all the signing hashes. So, it's fine to lose
the key pair on target delete.

src/test/backups-test.js

@@ -28,7 +28,8 @@ describe('backups', function () {
         preserveSecs: 0,
         label: '',
         appConfig: null,
-        targetId: null
+        targetId: null,
+        integrity: null
     };
 
     const appBackup = {
@@ -44,7 +45,8 @@ describe('backups', function () {
         preserveSecs: 0,
         label: '',
         appConfig: null,
-        targetId: null
+        targetId: null,
+        integrity: null
     };
 
     let defaultBackupTarget;