[FIX] sale: fix context update wrongly migrated / updated at...

[FIX] sale: fix context update wrongly migrated / updated at 94716a3f

Purpose of the code is to automatically add the recipients of quotation emails
as followers. It is done using the mail_post_autofollow context key. However
current code add the context key in all cases and not just when composing
a message on sale order.