[FIX] web: no chrome autocompete on many2one widget

For editing many2one fields (and partially many2many) most widget show
an autocompleting list of targeted records.

They thus have `autocomplete="off"` to prevent browser completion.

But chromium has an history of breaking `autocomplete="off"`, see:

- https://caniuse.com/#search=autocomplete
- https://crbug.com/468153
- https://crbug.com/587466
- https://crbug.com/914451
- https://crbug.com/923895

It seems that since chromium 71, the heuristic to ignore
`autocomplete="off"` has become more aggressive and for example if there
is at least 3 fields like an address in a page, chromium will ignore
`autocomplete="off"` for the fields like an address.

So for example the eidting the many2One field with placeholder "Country"
in a contact page now has a browser autocomplete menu that is:

- hidding the many2one autocomplete
- going to save empty country it appeared visually filled if the
  autocomplete result was selected.

With this changeset, the placeholder in the many2one instance is
interspersed with U+FEFF charcters (ZERO WIDTH NO-BREAK SPACE) so the
browser does enable the autocomplete feature by force.

This should thus remove the issue (until it is fixed by chromium) in the
case of field named "Country" or matching other regexes in this file:

https://github.com/chromium/chromium/blob/cdb1b2073f12/components/autofill/core/common/autofill_regex_constants.cc

U+FEFF has been chosen instead of more recommended characters because
other have been shown erroneous for printing in some windows
configuration (see cb2a3af).

10.0 version of odoo#30439
opw-1930588
closes odoo#30439
closes odoo#30449

addons/web/static/src/js/views/form_relational_widgets.js

@@ -264,6 +264,12 @@ var FieldMany2One = common.AbstractField.extend(common.CompletionFieldMixin, com
             }
         });
 
+        // avoid ignoring autocomplete="off" by obfuscating placeholder, see #30439
+        if (this.$input.attr('placeholder')) {
+            this.$input.attr('placeholder', function (index, val) {
+                return val.split('').join('\ufeff');
+            });
+        }
         var isSelecting = false;
         // autocomplete
         this.$input.autocomplete({