Add field to skip backup for an app

This skips the app from a backup when doing a full box backup and
simply reuses the previous backup.

The app can still be explicitly backed up using 'cloudron backup'
and explicitly restored using 'cloudron restore --backup'.

When restoring the box, it all depends on the app's last backup.

Fixes #311

migrations/schema.sql

@@ -69,6 +69,7 @@ CREATE TABLE IF NOT EXISTS apps(
     sso BOOLEAN DEFAULT 1, // whether user chose to enable SSO
     debugModeJson TEXT, // options for development mode
     robotsTxt TEXT,
+    enableBackup BOOLEAN DEFAULT 1,
 
     // the following fields do not belong here, they can be removed when we use a queue for apptask
     lastBackupId VARCHAR(128), // used to pass backupId to restore from to apptask