cubicweb-expense #344315 fix currency handling [validation pending]

We have two currency fields : one for the amount paid and one for the VAT, and only one conversion rate.

It is very unclear what happens if I input a line paid in one currency and taxed in another currency. Use case at hand : I pay in euros in a swiss restaurant. The bill has the amount in euros and the VAT in CHF. In the total amount for the VAT, the exchange rate is not used and the CHFs are summed with the Euros.

After testing a bit, the conversion rate seems to never be used in the primary view of an expense. And the generated PDF is horribly wrong.

done in0.4.4
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