sourceMigration() dropped when editing fields

The new field mapping editor tries to provide a reasonably simple interface - i.e., not presenting every possible option/subfield, but attempting to override any code-defined mappings intelligently. One rule is that if the source field and default value selected in the UI match a mapping from the underlying code, the mapping is not saved in the database so the full mapping info provided by the code (including in particular any sourceMigration) will take effect. Somewhere since the initial implementation this broke, so in particular migrate_d2d node migrations lost the sourceMigration on their uid mappings and authors were not properly migrated. Fix to come shortly...