PURPOSE

Allow the user to enable pipeline staging on any model with studio.

Idea: forget about m2o stage_id and always use selection field (both from configurator and formview)
    either the user is enabling the stages from the configurator at the creation of a new model...
        after submitting the configurator, he is redirected to the formview of the new model
        this formview has a stage status bar with default 'new/in progress/done' stages
        to edit this, his first instinct will be to try and edit the m2o field, but this won't work as those are records not selection values
        we can't expect him to close studio and go to the kanban view to edit his pipeline stages
        he could also close studio and go to the configuration>stages menu item, but this is just as confusing

    ...or he's enabling the stages from the formview of an existing model (which sets up a selection field)
        and, since there is no group_expand method defined on the selection field, only non-empty groups are actually showing up (defeating the purpose of 'pipeline staging')
        plus, we can't have a different behavior than with the configurator

    the reason why we use m2o stage_id in crm/project/helpdesk is because:
        it's great for onboarding, as the user can set up his own stages when discovering the app
            but this is not necessary here as the user is already customizing his model with studio
        we want to be able to define which project/salesteam has which stages, which wouldn't be possible with a selection field
            it is irrelevant when doing it through studio as the group_expand method is basic and will get all the stages
        we can afford to let several people create stages on the fly as those stages are by default restricted to a specific project, therefore not creating any noise for other users
            we can't afford to do that with custom stages models, meaning that we need to restrict the access rights to admins.. might as well do it through studio then

SPECIFICATION

In short, when setting up custom pipeline staging, instead of using a stage_id many2one field and a new 'stage' comodel, we are now using a simple selection field.

-> Enabling pipeline stages from the studio model configurator

Instead of creating a new stage_id field and its corresponding comodel

    create a selection field with the following values: 'New','In progress', and 'Done'
    set up this field with widget=statusbar in the formview header
    set 'New' as the default value
    activate the kanban view and set default_group_by=XXX
        where XXX is the pipeline selection field
    set group_expand=true on the selection field

Enabling pipeline stages from the formview

When adding the 'pipeline status bar' custom field with studio:

    enable the kanban view
    replace the selection values 'First/second/third status' by 'New', 'In progress', and 'Done'
    set the 'New' selection value as default
    set the selection field as 'default_group_by' in the kanban
    set group_expand=true on the selection field

In base.view_model_fields_form, remove the readonly and invisible modifiers on field group_expand.

TASK 2226581