Provide easy way to give extra panels at show-time.

[chmarr]

At show-time it is common to give artists additional panels to "spread out". Since this is the staff's decision the artist should not be charged for the panels.

However, this either not accounting for the panels in the "allocation", and remembering this fact if allocations are being reconciled against the artist-to-panel report, or allocating the panel and remembering to not charge for them when doing the "apply space fees", or creating a new category of space that has no cost, and then moving the panels from one category to another. MESSY!

Suggested solution: add additional attribute to a SpaceAllocation that indicates how many gratis spaces are being allocated. TBD: does the value add to the allocation, or is part of the allocation.

Bonus: This will nicely cover "GOH complimentary panels", removing the need to have a separate class for this, and possibly removing the need to have a "secret, un-reservable" category.

Extra: Possibly add another category for a space that is no longer allocated, but still being charged for. This will simplify "unused space penalty".