backups: add remotePath

the main motivation is that id can be used in REST API routes. previously,
the id was a path and this had a "/" in it. This made /api/v1/backups/:backupId
not work.

src/routes/test/backups-test.js

@@ -92,7 +92,7 @@ describe('Backups API', function () {
             const response = await superagent.post(`${serverUrl}/api/v1/backups/${someBackup.id}`)
                 .query({ access_token: owner.token })
                 .send({ preserveSecs: 30, label: 'NewOrleans' });
-            expect(response.statusCode).to.equal(400);
+            expect(response.statusCode).to.equal(200);
         });
     });
 });