[FIX] website_sale: set partner lang according to website lang

On ecommerce checkout, the language of the partner wasn't set according to the language in which he is visiting the website.
Therefore, its partner was set with the default language (English in most cases), and any emails sent to him were not translated in his own language (in the email templates, such as the quotation email he received on order confirmation)
[IMP] crm_partner_assign: add field assigned_partner_id in view

website_customer filters the partners based on assigned_partner_id (the list of
customers that were impletemented by another partner). This field was not
visible so no way to actually display a partner in this list.
Fixes #430, opw 619254
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64