[FIX] mail: thread messages order on show more

If click show more in a flat thread (level 0, such as under leads, issues, etc.) append messages from newer to older
If click show more in an indented thread (level > 0, such as in the messaging wall), append messages from older to newer
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32