archive: appConfig is null for pre-8.2 backups

use backups.manifest when possible instead

src/routes/test/archives-test.js

@@ -46,6 +46,7 @@ describe('Archives API', function () {
         expect(response.body.archives.length).to.be(1);
         expect(response.body.archives[0].id).to.be(archiveId);
         expect(response.body.archives[0].appConfig).to.eql(appBackup.appConfig);
+        expect(response.body.archives[0].manifest).to.eql(appBackup.manifest);
     });
 
     it('get valid archive', async function () {
@@ -53,6 +54,7 @@ describe('Archives API', function () {
             .query({ access_token: owner.token });
         expect(response.statusCode).to.equal(200);
         expect(response.body.appConfig).to.eql(appBackup.appConfig);
+        expect(response.body.manifest).to.eql(appBackup.manifest);
     });
 
     it('cannot get invalid archive', async function () {