[FIX] im_chat: unlink im chat presence on user deletion

It wasn't possible to delete a user if he had a im chat presence linked to him.
[REF] mail: extract auto subscribe notifications sending into a sub-method

To allow customization, such as do not send the notification to the partner that subscribed himself to the thread (he probably already read the thread, no need to send him an email)
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32