Purpose
=======

In eLearning you currently have to create a certification before adding it to
a course in front-end. This PR allows creation of certification in front-end
with pre-filled settings.

Functional Spec
===============

1. On a course, add content, type certification, you can choose an existing one
   (normal flow) or create a new one in the dropdown by typing a new survey
   name (new flow).
3. When submitting (Publish or save as draft), a certification survey, with
   pre-filled settings is created and linked to the slide.
   Pre-filled settings:
        * One page per question
        * Attempts limit : 1
        * Time limit : no limit
        * Scoring without answers
        * Passing score : 70%
        * Is Certification : True
        * Email Template : Certification: Send by email
        * Give Badge : False
4. After submit, the user stays on the Course Content.
5. A toaster is displayed in the top right saying:
    Toaster's Title: Certification created
    Content: Follow this link to add questions to your certification.
    <a>Edit certification</a>

A website editor can select a certification (if available) for the creation of
a slide.

This PR also includes various IMP for a better user experience as well as
some code cleaning

  * use badge level image as default badge image
  * force duration to be positive in upload slide widget
  * clean slide upload widget and controller code

See sub commits for more details.

Link
====

Task ID 1981399
PR odoo/odoo#43577