DB storage of field mappings

To support #1826112: Wizard framework, we need to store field mappings made through the UI, to be applied when instantiating migrations thus dynamically defined. So, we need a db table to store them, and to work out how db-defined field mappings related to code-defined field mappings, with an eye towards ultimately having migrations as features (i.e., exporting a migration definition configured through the UI as code, overrridable through the UI).