[FIX] website: review t-cache of the <head> in website.layout

The `t-cache` used on the `<head>` of website.layout was wrong: it
depended on the use of debug=assets or not (added by [1]) while it
should at least also depend on debug=tests. This commit chooses to fix
the issue by not using the caching system in case any value of debug
is used.

This commit also comments this `t-cache` value.

Steps to reproduce:
- As a visitor, first visit the homepage without debug mode
- Now reach the homepage with ?debug=tests
=> You still don't have the tests assets

Worse (but unlikely in production of course):
- Create a fresh DB and first reach the homepage with debug=tests
=> Now all your visitors are damned to load the tests tours for no
   reason (and you likely have lots of errors shown in your console).

[1]: https://github.com/odoo/odoo/commit/3061a484168ff21e3ae7e40ce691d299b63484a4

opw-3203912

closes odoo/odoo#114875

X-original-commit: 5efb637d880f82ea81bb64a0d091ffa7bdf7635b
Signed-off-by: Romain Derie (rde) <rde@odoo.com>
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/website/views/website_templates.xml

@@ -55,7 +55,20 @@
     </xpath>
 
     <xpath expr="//head" position="attributes">
-        <attribute name="t-cache">('assets' in request.session.debug, main_object) if main_object and main_object._name == 'website.page' and not (editable or translatable) else None</attribute>
+        <!--
+        Cache the <head> of the website.layout:
+        - For each individual website.page record (their css/js might be the
+          same but their title will not for example). Arguably we could do this
+          for any record (not only website.page) but this t-cache was originally
+          added to replace the website.page own cache system. For other records,
+          it could probably depend on each individual access right.
+        - Only for non-designers (especially visitors) (hence the "editable or
+          translatable" check).
+        - Only if not in any debug mode (we want any correct value of debug mode
+          to correctly appear in the `odoo` object + assets may depend on this
+          (e.g. debug=assets or debug=tests)).
+        -->
+        <attribute name="t-cache">main_object if main_object and main_object._name == 'website.page' and not request.session.debug and not editable and not translatable else None</attribute>
     </xpath>
     <xpath expr="//head/*[1]" position="before">
         <t t-if="not title">