Use backend isAvailable status for pickup display

Backend already tells if it would accept a signup for a pickup (mostly at least), use this to display the appropriate state.

I think I am not writing changelogs for these MRs because it is just a followup to an unreleased feature (and I expect complains about a pickup with an error message tomorrow (because of !945 (merged)), if this does not get merged before :D)

Related? #633 (closed)