[IMP] mail: ease the use and specification of notification emails layout

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).

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