Consider visible custom fields in compute_final_field_selection so that
a link with only custom_field_X is not wrongly treated as invalid and
reverted to session/global. Add test for URL containing only custom field.
Append membership_fee_type to column list when it is visible but
membership_fee_start_date was not in the selection (MemberExport,
export_column_order, build_export_member_fields_list).
Use Ash related-field sort (membership_fee_type.name) instead of
membership_fee_type_id so column order is alphabetical. Load
membership_fee_type when sorting by it even if column is hidden.
In-memory re-sort (Build) uses loaded fee type name.
Read 'fields' from URI when conn.params has no query (e.g. full page load).
When ?fields=... is present use URL-only selection so columns are not
merged with global settings. Fall back to session+global when URL has
only invalid field names.
