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/apptask.js

@@ -566,14 +566,18 @@ async function updateCommand(app, args, progressCallback) {
 
     if (!updateConfig.skipBackup) {
         await progressCallback({ percent: 15, message: 'Backing up app' });
-        const backupSite = await backupSites.getPrimary();
-        // preserve update backups for 3 weeks
-        const [error] = await safe(backuptask.backupApp(app, backupSite, { preserveSecs: 3*7*24*60*60 }, (progress) => {
-            progressCallback({ percent: 15, message: `Backup - ${progress.message}` });
-        }));
-        if (error) {
-            error.backupError = true;
-            throw error;
+        const sites = await backupSites.listByContentForUpdates(app.id);
+        if (sites.length === 0) throw new BoxError(BoxError.BAD_STATE, 'App has no backup site for updates', { backupError: true });
+
+        for (const site of sites) {
+            // preserve update backups for 3 weeks
+            const [error] = await safe(backuptask.backupApp(app, site, { preserveSecs: 3*7*24*60*60 }, (progress) => {
+                progressCallback({ percent: 15, message: `Backup - ${progress.message}` });
+            }));
+            if (error) {
+                error.backupError = true;
+                throw error;
+            }
         }
     }