[FIX] web_editor: ol in ol no number skip

In the case of an <ol> in an <ol> we would have something like:

```
1.
   1.
   2.
3.
```

because the sublist itself in version of summernote.org we are using
is a list item, so it takes one extraenous item.

With this changeset we have something that makes more sense:

```
1.
   1.
   2.
2.
```

Because there are tons of use case of different selection of list level
and indent or outdent, this commit also simplify outdenting (eg. now we
outdent all the element of the same level than the first indented).

opw-2185406
closes #48194

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>