[FIX] web_editor: resolve ajax request of cleanForSave

Upon save in website or in mass_mailing, if a `cleanForSave` of a
`SnippetEditor` or a `SnippetOption` wait for the end of an
`this._rpc()` request AND the `SnippetEditor` or the `SnippetEditor`
of the `SnippetOption` has a `SnippetEditor` ancestor, the
`cleanForSave` will never resolve.

Here is the scenario:

- **SnippetEditor B** has **SnippetEditor A** as its odoo widget parent

In `SnippetMenu` `_destroyEditors`:
- **SnippetEditor A** and **all its SnippetOption** call `cleanForSave`. 
- **SnippetEditor B** and **all its SnippetOption** call `cleanForSave`.
  One of the promise of **SnippetEditor B** includes the result of
  `rpc()` in `ajax_service.js`.

- **SnippetEditor A** and **all its SnippetOption**
  `cleanForSave promises` resolve (but not **SnippetEditor B**).
- **SnippetEditor A** get's destroyed (`snippetEditor.destroy()`). As
**SnippetEditor B** is a `Widget` child of **SnippetEditor A**:
- **SnippetEditor B** get's destroyed (here is where the problem start).

... in the mean time ...

The ajax request of a promise of **SnippetEditor B** ends. The promise
of the `rpc()` will never finish because **SnippetEditor B** is
destroyed and the condition in `rpc()` is:
```js
if (!target.isDestroyed()) {
    resolve(result);
}
```

Because the promise will never finish for `_destroyEditors`, the save
action will never fully complete.

Task-2742008