[Spaces] Grace period before disabling / deleting Spaces

Disabling or deleting a Space could cause serious problems for the clients, for example with currently running uploads. (dataloss!!)

I want to propose a grace period. Whenever an admin disables or deletes a Space in the web UI, a dialogue should open with a date picker, at which date the Space will be really disabled or deleted.

During the grace period, clients should tell their users, that the Space will be disabled or deleted soon:

/cc @TheOneRing @felix-schwarz @kulmann @micbar @tbsbdr @pmaier1