controllers: avoid setting constants as controller instance variables in __before__

Setting constants in __before__ is a weird pattern and we can do fine without doing it.
That makes it more clear that there is no state in the controller instances.

Real constants can just be set at the module level.

Some values depend on configuration and can thus probably not be set as
constants at module import time but could perhaps be set in __init__. But
reading configuration directly when needed will probably be just as good.