[REF] survey: convert survey.page to survey.question -> is_page=True

Task #1902306

Purpose
=======

survey.question is now also the model used for the survey's pages (with the "is_page" field set to True).
This allows to put all the pages and questions together in a o2m field on the view side and
easily reorganize your survey by dragging the items around.

It also removes one level of encoding by directly having 'Add a page' and 'Add a question'
links on the tree view of questions, enabling a faster encoding.

However, this has the downside of making the code reading a little bit more complicated.
Efforts were made at the model level to create computed fields so that the use of these models
still seems somewhat logical. That means:
- A survey still has "page_ids" (question_and_page_ids filtered on is_page = True)
- These "page_ids" still have question_ids (questions located between this page and the next)
- These "question_ids" still have a "page_id"

That makes the use and display of these information at view and controller levels easier to understand.

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