backups: add per site enabledForUpdates flag

previously, we had a singleton 'main' flag to indicate a site can
be used for updates. with this new approach, we can get rid of the
'primary' concept. each site can be used for updates or not.

src/test/common.js

@@ -222,16 +222,7 @@ async function databaseSetup() {
     await dashboard._setLocation(constants.DASHBOARD_SUBDOMAIN, exports.dashboardDomain);
 
     // duplicated here since we clear the database
-    const id = await backupSites.add({
-        provider: 'filesystem',
-        name: 'Default',
-        config: { backupDir: '/tmp/boxtest' },
-        format: 'tgz',
-        retention: { keepWithinSecs: 2 * 24 * 60 * 60 },
-        schedule:  '00 00 23 * * *',
-        contents: null
-    }, auditSource);
-    await backupSites.setPrimary({ id }, auditSource);
+    await backupSites.addDefault(auditSource);
 }
 
 async function domainSetup() {