feat: coop management

[crosspolar]

I figured, it would be nice if Vorstand is not relying on us to change the address of SuperCoop :). could superseed #819

[Theophile-Madet]

I'm not sure it's worth the added complexity for something that changes every 5 years, but if we do want it, could we make it more generic?
Instead of a CoopAddress model we could have something like

class CustomizableString(models.Model):
    key = models.CharField(...)
    value = models.CharField(...)

Where the list of keys is defined as constants, and the keys are coop_name, coop_full_name...
That would let us add more strings by adding a constant and we're not bound to the CoopAddress name so we can put other things in it.