Visible sold-out condititions

[jamezpolley]

Currently, if an item is unavailable, Symposion simply does not show it.

This makes sense in some cases (eg, for a "Speaker ticket" that should only be visible to speakers)

However, in some cases - particularly in cases where there were a limited number of something and that cap has been reached - removing the item leads to confusion. People can't tell the difference between "I have registered too late and there are no more t-shirts of my size available" and "WTF the organisers have forgotten to sell shirts in my size!"

It would be nice - in some cases - to be able to continue to list the sold-out options, but flag them in some way as being sold out.

This interacts a bit with #136 : if A is sold out, in some scenarios, it might make sense to have product A-Waitlist become available.