[FIX] website: support RTL for the opening direction of the sub-menus

Since [this commit], some JS code tries to open the sub-menus in the
right direction so that the page overflow is limited. Unfortunately,
languages that read from right to left (RTL) were not supported by this
code, which left some sub-menus of the navbar overflowing when there was
room to open them without overflowing (on the other side).
Steps to reproduce the bug:
- Have a sub-menu in the last but one position of the navbar with long
strings of characters.
- Align navbar items to the right.
- Have an RTL language installed on your website.

=> When you open the sub-menu in RTL, it opens on the wrong side and
causes an overflow of the page.

[this commit]: https://github.com/odoo/odoo/commit/392c91cda3133b921e9aca9a7b1c511231027438

task-2904507

Part-of: odoo/odoo#115153