Exchanges including VAT for non-EU orders

[irisSchaffer]

What does this do?

Updates Return Assembler to calculate the exchange item's price details (net, gross, tax, actual price etc.) correctly and to set the balance for exchanges for non-EU orders to the exchange item's gross - the returned item's gross. Made front end account controller use the return assembler instead of putting the form data into the session (this is how it works with orders and with epos returns already). This means the data shown to the user and the data in the session(return assembler) will now always be the same and we don't do the same stuff in two places anymore.

How should this be manually tested?

Go through the return steps for EU and non-EU orders, check whether the front end still looks how it should. Check whether the balance displayed is correct (and the same as the value saved to database). Have a look at EPOS to make sure nothing breaks there.

Also check the e-mails sent when you confirm the balance.

Related PRs / Issues / Resources?

Relies on #162 Original issue: #166 This fixes #166 and closes https://github.com/messagedigital/cog-mothership-returns/issues/151

Anything else to add? (Screenshots, background context, etc)

[thomasjthomasj]

When exchanging for an item of the same value outside the EU, the price appeared to be correct in the admin panel but when submitting the return I got this:

[thomasjthomasj]

When exchanging a car coat (£291.67 with VAT removed) for a pocket square (£50 with VAT still on it), I was told I would receive £241.67 when submitting my return, but in the admin panel it says I'm owed £300. Neither of these are correct! If my sums are correct, it should be £250.

[thomasjthomasj]

Same deal when exchanging for something more expensive. Swapping the above car coat for a double weave jacket (£475 w/ VAT, £395.83 w/o), it says I need to pay £183.33, but it should be £154.16, and then the customer balance in the admin panel is saying it's £125

[thomasjthomasj]

Ich bin ein numpty