Add date and age to dunning

From https://discuss.tryton.org/t/send-an-email-automatically-passed-some-days-after-processing-the-dunning/4591 it may be useful to know the age of a dunning.

assigned to @ced

added accounting type::feature + 1 deleted label

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New review352661002 at https://codereview.tryton.org/352661002/#ps371761002

review352661002 updated at https://codereview.tryton.org/352661002/#ps352721002

New changeset 7127d66100b8 by Cédric Krier in branch 'default':
Add date and age to dunning
https://hg.tryton.org/modules/account_dunning/rev/7127d66100b8

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closed

New changeset 58596bd31b5b by Cédric Krier in branch 'default':
Add date and age to dunning
https://hg.tryton.org/tryton-env/rev/58596bd31b5b