Recipient list: Fix improper location of width style, fix the tabindex and use a proper label
Various additions to ajax/sendMessage.php

* Show an error when no recipients are specified
* Add currently logged-in user to the list of recipients
* If a new group was created, output its ID
ajax/sendMessage.php: Fix check for empty recipient list
Add support for sending new messages via AJAX
Make the ID of the recipients list more consistent to other IDs