Add fallback and use mechanism usage

Like we did for notification_email, it is good to have a fallback to process the email manually. Also we should allow to define which email to use.

assigned to @ced

added accounting type::feature + 1 deleted label

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review70311002 updated at https://codereview.tryton.org/70311002/#ps1

New changeset 075d0152feb8 by Cédric Krier in branch 'default':
Add fall-back and use mechanism usage
https://hg.tryton.org/modules/account_dunning_email/rev/075d0152feb8

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closed

New changeset b898ce9cd769 by Cédric Krier in branch 'default':
Add fall-back and use mechanism usage
https://hg.tryton.org/tryton-env/rev/b898ce9cd769