corporations: do not include officers_parent in corporation groups.

trello: https://trello.com/c/NlkOMW3k/484-fix-amtstrager
user import: fixing aktivitätszahl issues: (1) correcting for missing space in netenv_aktivitätszahl. (2) multiple memberships beginning in the same year have to keep the order of the aktivitätszahl correct.
corporations: trying to filter out the officers_parent group that might be a child of the corporations_parent. For some reason that does not work as expected. Please use Corporation#all if you need just the corporations without the officers_parent.
corporations: clean-up after trying to fix Group#corporation_groups.
changing error message.

trello: https://trello.com/c/9cNKrtwO/476-fehler-seite-korrigieren
user import: Kürzel der schweizer Verbindungen anpassen. Offenbar gehört 'Z' doch zu Zürich.
user import: Fehlende Leerzeichen in netenv_aktivitätszahl zwischen Verbindungskürzel und Jahreszahl ergänzen.