[FIX] models: custom fields seen as base fields

This was possible to create custom fields `x_*`
but seen as base fields.

For instance,
 - Go to Settings > Technical > Database Structure > Fields
 - Select a field (any)
 - Click on the model link, to be redirected to the model form
 - Edit & add a custom field from there.
 - Save
 - Notice that the field you just added is saved as a base field.

We solve this issue by assuming that all created fields and models
are customs, except the ones created by the ORM, by the database
initialization, the fields coming from the modules in python.

We therefore remove the mechanism on which a field was set
as custom according to the fact `manual` was set to True within
the context: This is now the case by default.

No change was required for the base fields: The `state` `base`
was already forced for those fields, that are created using
direct SQL requests `INSERT INTO`.

opw-657312