[FIX] website, website_report: backwards compatibility with outdated templates

* Complements commits a6969133 and
21d4b3fd by adding the missing `data-lang`
attribute also in the report layout (in saas-6 a single QWeb
template is used for language links in both reports and website
layouts).

* Fix the "Edit Master" link to work also for outdated templates
where the data-lang attribute is dynamically set to 'default',
and thus cannot be used as URL prefix -> use /website/lang
controller to switch lang instead.