Add -app prefix for all app sources

so that this doesn't conflict with some user.

src/addons.js

@@ -391,7 +391,7 @@ function setupSendMail(app, options, callback) {
     var env = [
         'MAIL_SMTP_SERVER=mail',
         'MAIL_SMTP_PORT=2500', // if you change this, change the mail container
-        'MAIL_SMTP_USERNAME=' + (app.location || app.id), // use app.id for bare domains
+        'MAIL_SMTP_USERNAME=' + (app.location || app.id) + '-app', // use app.id for bare domains
         'MAIL_DOMAIN=' + config.fqdn()
     ];