officers: prelim ui to add new offices.
Only global admins can use this for the moment.

trello: https://trello.com/c/1UqSsBHx/681-neue-rechte
rake patch:officers

* fixing issues
* naming groups (with names, they can be found via search)
* recalculating validity ranges

trello: https://trello.com/c/oriKIGdx/693-amtstrager-struktur-ausbauen
fix: Verstorbene behalten ihre Aktivitätszahl.
Structureable#find_officers_groups: do not create the officers parent.
DagLink#delete_cache

also delete cache for indirectly linked objects. If this takes too much
time, we need to rethink this.