[IMP] don't pointlessly recreate view_obj every time _views_get is called, better use of pool & data APIs

should probably validate that there's an inherit_id (?)

Not used yet, only defined its relation to inherit_id:

not inherit_id + primary -> ok
not inherit_id + extension -> error
inherit_id + primary -> ok
inherit_id + extension -> ok

[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