Feature request: retain extra fields

Not sure whether this is a bug or a feature, but it seems that enabling ckanext-scheming with a custom schema will, on first saving a dataset with the new schema, delete any existing "extra" fields. Even to only retain the existing extra fields to safely migrate data would be helpful - this could be achieved if config settings would toggle the display of existing extra fields and/or form fields.

Start by reproducing the first dataset save with ckanext-scheming enabled and a custom schema, then trace where existing "extra" fields are removed. Done means existing extra fields are retained during migration, with any configuration for displaying them or their form fields defined and verified.

Written by the indexing model from the issue text.