[FIX] account_anglo_saxon: price difference, discount and taxes

When computing the price difference amount do not integrate the eventual discount and taxes included in the price.
Otherwise the total of the generated accounting enty would be higher than the total of the invoice. opw 611350
[FIX] event: missing context

This fixes some translation issues (templates not translated at event confirmation)
[FIX] event: double context
Forward port of branch 7.0 up to b3485fb