Don't overwrite accepted_at when importing clients/volunteers
https://trello.com/c/IHvNLfQp/203-05-implement-map-access-rollenbeginn-to-acceptedat-field-on-our-db
The values were already mapped, but in Volunteer it was overwritten by the record_acceptance_change callback.
The values are also already displayed on volunteers#edit below the dropdown:
@vok should we maybe display them on #show as well?
Edited by Markus Koller