This checkbox caused greate confusion, as it was doubled in the tax form view. Field is already present in line 937

Thanks for merging.

* document_page should have disappeared a long time ago, no idea why it
  remains
* thunderbird seems to have suffered from odd conflictual moves (or lack
  thereof)
* audittrail was removed but not deactivated in rosetta

[FIX] website:  ir_http: when rerouting url containing default language -> url without language, the ? was missing, leading to wrong urls.

[FIX] website: define languages variable outside of an 'if' as it is used below in the same template, leading to a crash due to the variable not always being defined.

Fixed checkout redirection that was meant to clean the session (incorrect sale order or transaction) that may happen
when having issues with the payment.

add the possibility to users to follow a forum and be automatically followers of new
questions, using some new subtypes on the forum.forum model that do the auto subscription.

Also added a profile link on the forum, when logged.

Also added a subscribe button on the forum.

[FIX] remove closing reason 'an answer was accepted', the questions should be left as open to be possible to improve the answer afterward

[FIX] website_forum: when sending emails about forum post, use the public url instead of the private

Also added a breadcrumb in partners navigation; when going on the partner view
a link to come back to the list view has been added and takes into account
selected grade and country.