Add export order merging feature to the "Manage Orders" page

[gl636474]

... which shows the cost prices of each products in each order together with the sale price and margin in pounds and percent for each item in the order and the order as a whole.

[gl636474]

May have to add an observer and/or table to record the cost price at the time of sale for each item in each order. The rest can be calculated.

[gl636474]

Jens wants to add columns for margin in pounds and percent to the table in the "Manage Orders" page AND to the "Items Ordered" table within the order itself.

A discount can then be easily determined either on the whole order itself or for just one of the items within the order (e.g. "If you make that order I'll give you 10% off the baked beans").