[IMP] account, sale, l10n_ch, purchase: replace mail composer inheritance by message_post override

Currently composer is inherited in those four modules to add a specific
behavior when sending the invoice / quotation by email. Purpose is to
mark it as sent, using a magic context key.

This commit keeps the feature but changes the implementation. It is now
done by overriding message_post which should be the entry point for all
message-related features.