[FIX] website: restore the redirect after lang install

Since 2013 we have a kind of a URL hack that allows to redirect to the
initial URL after installing a lang through the website, see [1].

It was even tested in python unit test with [2] but sadly it was still
broken by a combination of [3] and [4] which actually broke it at the
javascript level: the python view was correctly still outputting the
correct URL and the python part was correctly handling that return URL
when passed, but the javascript was not actually passing the URL from
the view/href to the python side as the lang install wizard was now
called through JS instead of a normal backend URL redirect (since the
website frontend > backend imp at [4]).

Step to reproduce:
- Go to any page other than the homepage
- Click on footer > add a language
- Add a language
- You are redirected to the homepage always instead of the URL you came
  from

Note that this is only true if done through the backend / iframe preview
as clicking on the "Add a language" from the frontend will work as it
should. Indeed, when clicking on it from the frontend, it's a simple
link redirect but when click in the iframe preview, the click is
preventend to let the JS open the action in the top window instead.

[1]: https://github.com/odoo/odoo/commit/5cfbcc3aff5a28022b397ec7a28ebaca6db43673
[2]: https://github.com/odoo/odoo/commit/269aa594111a152ad4b7714856ea745bfef57155
[3]: https://github.com/odoo/odoo/commit/11429329b8dea2dd0b2496dc8c1c8627751cccff
[4]: https://github.com/odoo/odoo/commit/31cc10b91dc7762e23b4bde9b945be0c4ce3fe3b

closes odoo/odoo#108869

X-original-commit: 05caf6e697461176326ee61beeccb1193472267c
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Signed-off-by: Romain Derie (rde) <rde@odoo.com>

addons/website/static/src/client_actions/website_preview/website_preview.js

@@ -353,7 +353,7 @@ export class WebsitePreview extends Component {
                     additionalContext: {
                         params: {
                             website_id: this.websiteId,
-                            url_return: '/[lang]',
+                            url_return: $.deparam(href).url_return,
                         },
                     },
                 });

addons/website/views/website_templates.xml

@@ -1929,6 +1929,8 @@
 </template>
 
 <template id="language_selector_add_language">
+    <!-- When clicked from the iframe preview in backend, the click will be
+    prevented to directly call the action in the top window -->
     <a t-attf-class="o_add_language d-none #{dropdown and 'd-sm-block dropdown-item' or 'd-sm-inline-block list-inline-item'}"
        groups="website.group_website_restricted_editor"
        t-attf-href="/web#action=base.action_view_base_language_install&amp;website_id=#{website.id if website else ''}&amp;url_return=#{quote_plus(url_for('', '[lang]') + '?' + keep_query())}">