* 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.

I understand the idea of not mixing controller function for multiple
things. but I also think that having post urls, which cannot be get
(even they do have a sensible meaning) is not nice.

especially for the signup process, I would assume, that if you hit
enter in the url bar, you would loose all data in the form, but still
get to the same signup page.

therefore i added the token as part of the url and the post goes to
'/signup/:token'. when you get that url, you can restart signup
with the same token.