[FIX] mail: make guideline template a qweb view

The mail template mail_template_channel_send_guidelines should only be used
from send_guidelines action. Anyway, with mail_template on activity
and all demo done to show this mechanic on contacts,
trying to render this one will generate a traceback. This fix
transforms the mail_template to qweb view so that it won't be available in
mail template.