[IMP] website_sale: correctly submit the payment form using a post request, correctly create the tx asynchronously

[IMP] Move website assets to frontend bundle

[IMP] payment addons: fixed submit buttons: now have type=submit (and not image or nothing) and removed name, because this may cause issues when trying to submit a form manually with type=submit and a name. Also improved js code in website_sale to correctly handle the click binding, the asynchronous call to the server to create the transaction, and then submit the form. As form submission is synchronous in javascript, we cannot use the deferred. Instead we prevent the click event from bubbling, wait for the server to create the transaction, adn then manually submit the form.

Now: intercept the call, make an asynchronous call to create the tx, then continue to the acquirer. Previous this was done by
a redirection, leading to a get request beign done. However it should be a post request.

[IMP] When no result is displayed in m2o field with option "no_create", add a "No result" value to avoid confusion.

* Deleted: manually translation in es.po

[IMP] Add 'Import Module' wizard to base_import_module

Removed confusing doubled check box "child_depend" in the tax view

[FIX] sitemap.xml urls

[FIX] website_forum: display the users as a table in block of 3 cols

Add contributing.md file

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

Thanks for merging.

Various ex-module remains

* 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: fixed languages errors
- ir_http: added a missing ? when a route using the default language was re-rerouted to remove the language part of the path, leading to a wrong url
- website layout: fixed use of a languages variable not always defined depending on the working conditions

[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.

Merge branch 'master-remove-html-fields-backend-ypa' of https://github.com/odoo-dev/odoo into odoo-dev-master-remove-html-fields-backend-ypa

[FIX] Issues #167. Remove obsolete shop reference