[FIX] web: 2many race condition on fast click on Save / Edit

For `this.viewmanager.active_view` to be set, the view manager
switch mode must be completed, meaning the deferred returned by
the function `switch_mode` must be resolved
(see `this.active_view = view;` in `view_manager.js`, line 146)

In a 2many field, the view manager `switch_mode` deferred is resolved
when `is_loaded` of this field is as well resolved.

Before this revision, clicking fast on Save then Edit
of a vendor bills resulted to a JS error due to a race condition,
because `this.viewmanager.active_view` was undefined when
the field was not loaded / the view manager switch mode not completed.

opw-678097