[FIX] web: notebook first page and invisible

When two first notebook pages are invisible, the shown page will be the
second invisible one.

With this fix, all oe_form_invisible classes are added in the init stage
of the NotebookInvisibilityChanger object.

opw-646740

note: good for saas-6 but not next versions, see opw-639595

addons/web/static/src/js/view_form.js

@@ -1808,6 +1808,7 @@ instance.web.form.NotebookInvisibilityChanger = instance.web.form.InvisibilityCh
     // called again when view content will be loaded (event view_content_has_changed)
     start: function() {
         this.on("change:effective_invisible", this, this._check_visibility);
+        this.$el.toggleClass('oe_form_invisible', this.get("effective_invisible"));
     },
     _check_visibility: function() {
         this._super();