[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
[FIX] widget html: rendering issue in mail composer on Firefox
The cleditor width does not include the margins. Setting 100% will make the editable area too large (104%) on Firefox (opw 611700).
This issue was already fixed in newer releases, from commit 9247c37de7d67beb479905e218656665dc12391f
Nevertheless, it introduced a new issue: while editing the form, the content of widgets html which were located in hidden notebook pages weren't displayed when displaying the page content.
opw-614448
[MERGE] forward port of branch 7.0 up to 9c77f79