[FIX] web_editor, website_form: speed up and robustify form editing

STEPS:
* open website page with a form (e.g. /contactus)
* click edit
* slow down connection speed
* click fast between form input lines or change order of the lines

BEFORE:
* 7 svg files are requested every click
* get_authorized_fields is called every click
* random error "'classList' of undefined" in SelectUserValueWidget::close
* random similar errors when a method is called before initialization

AFTER:
* svg and get_authorized_fields are requested once
* random errors are no reproduced or it's hard to do it
* performance of switching between inputs doesn't depend on connection
speed (though, still not instant)

WHY:
* about changing in SelectUserValueWidget::close: posible reason why
this.menuTogglerEl may be undefined is because method start is async
* the other updates just implement caching

---

opw-2368928

closes odoo/odoo#61598

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