OAuth Provider: Allow several/all Grant Flows

Currently, the form for registering applications wants to know which Grant Flow the client will use. This is very uncommon – I have not seens any other OAuth provider that even cares.

Please investigate how we can remove the need – if we can keep it as an option (i.e. allow selecting multiple Flows, with the default being all available Flows), then I would prefer this. If not, we can just always allow all Flows.