[IMP] website_mail: mailing editor buttons

Button "Save and continue" was wrongly named as it worked only once the template
is not in edit mode (so already saved).
Hide the button to only get it in readonly mode and rename it for better
understanding of its purpose. (opw 614563)
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32