This commit proposes to ease the use and specification of layout used to
encapsulate email body when sending notification emails. This is done
through two means
* message_post_with_template now supports the custom layout parameter
that allows to set the layout to use when sending notification emails
in the message_post process. We have to rely on the context as it is
the current way of using it in the composer. It is likely to change in
a near future;
* composer now supports through context (as we have no other choice)
specific notification values to give to the notification layout rendering;
This commit is related to task 51122 (and PR #24052).
Pratima Gupta
1184f7872d