[MERGE][FIX] livechat, *: open livechat from URL

*: mail, web

This fixes the following issues:

### 1. Livechat menu was not visible when accessing from the customer URL

The customer URL is obtainable by following those steps:

- Access `/web`
- Click on Live Chat app
- Open a Live Chat Channel
- Click on "Widget" sheet
- Access livechat from (2nd) "Copy Text" button

This is due to missing JS modules in the external lib, such as the root widget.

### 2. The position of the buttons "Copy Text" is modified on pressing 'ALT'

This is caused by the keyboard navigation overlay that changes the CSS
position of the parent of the overlay element. It sets the position to
`relative`, so that the overlay exactly covers the element. However, if the
element had a position `absolute`, changing it to `relative` may re-position it
somewhere else on the page.

In fact, it is not necessary to set the CSS position to `relative` in this case,
because the overlay has CSS position `absolute`: an element with `absolute`
will match the size of the parent if the parent is also `absolute`.

Task-ID 1885445

closes odoo/odoo#27578

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