[americanexpress] fix double-counted balance/coming
In case accounts is having one or several child cards linked to the AMEX account then balance/coming is double counted: Main card account takes global balance/coming which embeds other accounts amounts from child cards and each child card will then also hold their own balance/coming.
Solution is to rely on splitted summary to get balance/coming breakdown at card (weboob account) level using period 0 instead of period 1.
- Add a bug fix where Card summary payment label in english is not correct and thus not properly flagged
- Add a split of the monthly autopay to spread the amount across the cards according to outsanding balances of each cards
This MR fixes issue #343