fix: assign a valid_from to memberships, when user is assigned by the `<<` operator.

trello:
https://trello.com/c/FCPN48wA/648-bei-admins-user-unbedingt-valid-from-s
etzen
group<< operator: adding (and fixing) specs.

trello: https://trello.com/c/FCPN48wA/648-bei-admins-user-unbedingt-valid-from-setzen
refactoring workflows import task in order to prepare the import of former-members workflows.
workflows: adding definitions for workflows Streichung, Schlichter Austritt, Ehrenhafter Austritt, Dimissio i.p. and Exclusio.
workflows: fix: wrap all workflows in transactions.
worfklows: fix: execute allows parameters.
workflows: mount former-memberships workflows in aktivitas and philisterschaft.

If they are mounted in the corporation, they show up for former members
as well.
former-meber workflows: destroy user accounts and remove from groups

trello:
https://trello.com/c/WlNlsXyF/340-workflows-verstorbene-ehemalige
workflows: wrapping workflows in database transactions by updating the gem.

trello: https://trello.com/c/WlNlsXyF/340-workflows-verstorbene-ehemalige
workflows: changing the icon for former-meber workflows.