Revert a single field, rather than all mappings.

Please consider allowing mappings to be reverted on a field-by-field basis, rather than the current system of reverting all mappings to code all at once. With this change, it would be possible to specify mappings with the UI, and then, after feedback from the client, map a single field using code while preserving the other mappings already specified with the UI. This use case comes up when client feedback shows that a particular field needs a callback to adjust the imported data, and callbacks for a field mapping can currently only be specified in code.