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* account properties (such as is mail crypt enabled) need to be queried and cannot be provided by auth anymore (since auth is now done through login.i.c)
* if an account is an admin is now supplied by saml property

TODO: some initial session information missing, since we do not
auth ourselves anymore.

TODO2: store and pass sso ticken to the backend

This reverts commit 0e992f32.

Mainly this should give us a better ux, when we link to individual
settings, like the password change page.

* completely disable user routes in the admin interface, imho they don't provide any value other than potential confusion
* work through the domain & users controller and adapt certain rails patterns
* try to make sure forms are filled on error
* make sure we use the _path helpers where possible
* try to further unify how the routes look like
* rename IndexController to WelcomeController as index#index sounds weird (nonsense change)
* match formfields to the param name itself, forms could be cleaned up more
* make it possible to create a forward together with a mailbox, we do that sometimes
* introduce a feature flag toggle. tbi.