do not add app mailboxes to database

a) we don't allow .app pattern in database for aliases and mailboxes
b) the addons already know about app names separately

src/user.js

@@ -95,6 +95,7 @@ function validateUsername(username) {
     // +/- can be tricky in emails
     if (/[^a-zA-Z0-9.]/.test(username)) return new UserError(UserError.BAD_USERNAME, 'Username can only contain alphanumerals and dot');
 
+    // app emails are sent using the .app suffix
     if (username.indexOf('.app') !== -1) return new UserError(UserError.BAD_USERNAME, 'Username pattern is reserved for apps');
 
     return null;