ModelTrainer: If model metadata is in db, but pickle is not available, and flag is set, update foreign keys

_From @thcrock on July 19, 2017 22:18_

The current code for this path has a workaround. We should support the replace-and-update use case with a flag, and correctly update foreign keys.

_Copied from original issue: dssg/catwalk#3_

Start by locating the ModelTrainer code path for existing model metadata when the pickle is unavailable, then inspect the current workaround, flag handling, and foreign-key updates. The work is done when the flagged replace-and-update case correctly updates foreign keys without an available pickle; the issue names no files or tests, so the relevant coverage must be located in the repository.

Written by the indexing model from the issue text.