[IMP] mail: make notification layout a real parameter of message post

Purpose of this commit is to add a new parameter to message post that
is the layout used when sending notification emails. It will soon
replace the context use of custom_layout in some case. It will also
allow to control a bit the notification process from the message_post
API in more or less planned future mail updates.

This commit also slightly changes the way notification process handles
deleted or unknown rendering templates. Before this commit any missing
custom template was replaced by the base template. A missing base
template was leading to a crash. Now it simply logs a warning and avoid
crashing.