Remove Line Item currency attribute

[mamhoff]

Somehow, in the past, it was possible to have orders with line items
in multiple currencies. The code to remedy this (Order#homogenize_line_item_currencies)
disappeared in #635, and now we have a bunch of code in different places
that allows setting it, raises errors if it's not there, and so on.

This commit delegates the line items currency to the order.
Whenever people try setting a line item's currency, they get a
pretty deprecation warning telling them not to do so.

The error handling functionality that seems to be used in the order
importer is still present (ie: If we know an imported line item has
a currency different from the order, we'll error out).

This commit replaces all that behaviour with a simple delegate that
always obtains a line item's currency from it's order.

Adds a changelog entry.

I also refactored OrderContents#add_to_line_item, taking code from
MountainRoseHerbs/spree@461f975.

That method should now be much easier to understand.

[gmacdougall]

I do think that this makes sense. However, I am concerned about potentially destroying data that people were relying upon if they had customized their store. I would appreciate some more opinions on this one.

[mamhoff]

Closing in favour of #1007, which does not remove the column but makes handling line item currencies saner.