erp5_base: Some fixes to currency exchange lines
The new feature is to allow defining currency exchange lines without a stop date, to say "we use this exchange rate until I enter the next one".
Also add some missing tests to make sure only validated currency exchange lines/cells are applied and fix a minor bug that currency exchange cells are not reindexed when currency exchange line is validated.
Since this all this is similar, it was easier for me to make one merge request with everything.