patch users part 3: flush aktivitaetszahl cache.
adding maintenance mode.

start with  rake maintenance:start
end with    rake maintenance:end

more info: https://github.com/biola/turnout
trello: https://trello.com/c/TDHdlnZC/557-wartungs-modus
fixing navable selection in horizontal nav by introducing a new Role model.
horizontal nav: fixing category indicator background colors.
fixing issues in User#member_of? in order to make the specs green again.
horizontal nav: (1) adding specs, (2) adding caching.
adding mechanism to import additional bv mappings.

trello: https://trello.com/c/LkRl2ZqX/560-zusatzliche-bvs
adding User#mark_as_deceased(at: date) method.
quick hack: Corporation#leaf_groups should also accept groups with several officers groups.
fixing layout issue with mini map on profile page.

trello: https://trello.com/c/GQDGRJ6a/559-fix-darstellungsproblem-mit-karte
fix users#show view: make W-Nummer not editable at all.

trello: https://trello.com/c/gai68Eo4/561-fix-w-nummer-nicht-andern-konnen
adding spec to make sure the wbl abo is terminated on User#mark_as_deceased.
adding links to blog post titles.
fixing red blog post spec.
adding map to search results view.
fixing red search view spec (missing geocoder stub).
welcome email: add notice not to forward the password.

trello: https://trello.com/c/5Fb4sNas/552-in-willkommens-e-mails-hinweis-die-nachricht-nicht-weiterzuleiten-da-passwort-enthalt
remove membership button: the button should only be visible for admins in the groups table.
groups#show view: do not display former members, since this could be confusing.

trello: https://trello.com/c/hbd1NluO/553-fix-in-corporation-verstorbene-anzeigen-ausgetretene-nicht
removing the mailer background service, since it is not used, yet.