#29 Specify promotions at the department level

[Iain Sinclair]

It seems that the system has made allowance for this in
the promotion_map, but any promotions mapped at the
department level are never used.

As an extension to change#925213 it would be
straightforward to modify the Item class to read
department level promotions too and add them to the
promotions vector.

There should probably be some way to specify how item
and department level promotions interact with each
other, for example:

1. don't use item promotions if dept promotions exist
2. don't use dept promotions if item promotions exist
3. item level promotions over-ride dept level promotions
of the same kind (class), but promotions of different
classes can be used together
4. use both no matter what

something like that.