[FIX] im_livechat: open website livechat window

Before this commit, when opening a website livechat window,
it crashed with the following error message in the dev tools:

```
Cannot read property 'html' of `undefined`
```

The problem was caused by the rendering of the chat window header
before the chat window widget is even mounted in the DOM. This error
has been introduced by the migration from jQuery 1.12 to 3, in which
most occurrences of `$.Deferred` have been turned into `Promise` [1].

`Widget.appendTo()` returns a `Promise` (was `$.Deferred` before).
A `Promise` is always asynchronous, whereas a `$.Deferred` with
jQuery 1.12 may be either synchronous or asynchronous. In the case
of the website livechat window, the implementation with `$.Deferred`
was always synchronous, which is no longer the case now.

[1] https://github.com/odoo/odoo/commit/126fb33f6a74916c808c8e988f55faf82f0b5759

closes odoo/odoo#32168

Signed-off-by: Jérome Maes (jem) <jem@openerp.com>

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