[FIX] kanban: wrong condition to reset the dataset index

The dataset index should be reset either if the dataset is empty or the index is greater than the records size
[FIX] event: translate templates into user language

When sending an email of the registration/confirmation, the context was lost and the tempate sticked in en_US.
With the patch, the template will use the current user's language.
[IMP] stock: display translated selection value in message

When a user tried to delete a done or canceled picking, the error messages used to display the key of the selection field ('done' or 'cancel') which was surprising in other languages than English. This patch takes the string value of the selection field, keeping the context to get the translated value (opw 613068)
[FIX] purchase: keep fallback behaviour that was removed at rev 8973217

If no valid invocie is found on the purchase order line, use the price and currency of the line (coherent with help message)
[FIX] account_voucher: convertion rate of purchase receipt

The conversion of currencies in vouchers are made based on payment_rate_currency_id field (instead of usual currency_id).
This field was not present in the purchase view and not correctly set when we change the journal (and thus the currency) which was leading to rates of 1. With a rate of 1, the amount is equal to the currency amount (which is rarely correct). opw 611254
[FIX] auth_ldap: remove unused sql column
[FIX] account_voucher: import invoices, sign according to transaction type
Merge remote-tracking branch 'upstream/7.0' into 7.0