[FIX] hr_expense: Make expense sheet report multi currency coherent

Purpose
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Currently, if we want to print an expense report that has several currencies in its expense lines, the total amount is hidden on the report. Otherwise, it is simply summed.

That could lead to confusing uses cases. Example.
1 expense line of 92 dollars.
1 expense report expressed in euros.

In the report we will have
- 1 line of 92 dollars
- Total amount: 92 euros

Which is obviously wrong.

Specification
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Always display the amount + Compute the amount in the expense report currency, by converting the amount with the rate at the day the expense has been made.