[FIX] web_editor, *: remove automatic content on undo/redo

* website_event

Commit https://github.com/odoo/odoo/commit/a153ed42a09f8b7f5e0865112eb7d5affc22a353


solved a big problem which was that when an undo/redo is performed, the
whole DOM was reconstructed breaking all the JS relying on the old one.
For example, the latest blog posts which are dynamically loaded in JS
were not removed before saving since the JS relied on the old DOM... and
this broke the page because that dynamic content contained non-valid
XML markup. The solution was to destroy all JS widgets before applying
an undo/redo and rebuilding them all afterwards. Ideally this operation
should be done on the undo recording action but this would have a huge
flickering impact since many DOM would be destroyed each time the user
types text (flickering which is also bad on undo/redo but it is more
acceptable).

The problem now is the following: if a widget, like many, is declared
like this:

```
start: function () {
    this.$el.append(/* Some dynamic content on page loading */);
},
destroy: function () {
    this.$el.find(/* Dynamic content to remove */).remove();
},
```

Then it works in all standard cases: dynamic content is loaded on page
load and is removed when saving the editor. But this happens with the
undo/redo system:

1. The users types text, we record an undo, which is the whole page
   current DOM, containing all the dynamic contents.

2. The users hits CTRL-Z:

    a. We destroy all JS widgets, calling destroy, the dynamic content
       is removed from the page.

    b. We replace the whole DOM with the one that was saved. That one
       contains the dynamic content DOM.

    c. The JS widgets are recreated, calling start... creating the
       dynamic content again.

Result: the dynamic content appears duplicated. On save, depending on
how the destroy was implemented only the last generated content may be
removed or both... but in any case it appears duplicated during edition.

Hopefully, our current stable version do not contain that many dynamic
content so a perfect amelioration of all of this can be found in master.
As a fix, this commit introduces an extra step between (a) and (b):
we remove the dynamic content of the DOM-to-re-apply before applying it.
For this to work, widgets have to mark their dynamic content with the
class 'o_temp_auto_element' when creating it. They also must add the
content they replace on the 'data-temp-auto-element-original-content'
attribute.

closes odoo/odoo#43512

X-original-commit: eb4aac3a
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>