[REF] survey: globally rename survey.label model to survey.question.answer

PURPOSE

As new features are about to land in survey, notably live interactions [1]
and new survey building [2] performing a pre cleaning is necessary. In this
PR we clean survey models by: removing unnecessary fields, cleaning some code
and finally renaming models.

SPECIFICATIONS

In this commit we rename the long standing ``survey.label`` model. Indeed
a label is something before a question, like an input label. Labels in survey
are used for suggested answers and sometimes as rows for matrix answers.

After much thoughts we rename ``survey.label`` to ``survey.question.answer``.
It indicates this model holds answers. Moreover it is namespaced within the
``survey.question`` model name to avoid conflict with user input / user answer
model.

As model naming changes, some fields also evolve. In survey.question model

  * ``labels_id`` is renamed to ``suggested_answer_ids`` to indicate it is
    used to display suggested values to the user;
  * ``labels_id_2`` is renamed to ``matrix_row_ids`` to indicate it is used
    to generate the rows of the matrix-type question. A matrix is therefore
    done using ``matrix_row_ids`` for rows and ``suggested_answer_ids`` for
    columns which seems easier to understand;

In survey.question.answer (old survey.label) model

  * ``question_id_2`` is renamed to ``matrix_question_id`` to ease its
    understanding, notably that it is used for matrix questions;

LINKS

[0] Related to Task ID 2061901 (survey models cleaning and preparation)
[1] Task ID 1972640 (live interactions)
[2] Task ID 2119587 (new frontend for building surveys)

PR #40765