Make it easier to clean up results schema

_From @thcrock on May 23, 2017 19:51_

The full models table (and child tables) can get cluttered with aborted/buggy runs.

We should make it easier to delete all the results schema data that we've decided is bad.

This is partially done in https://github.com/dssg/results-schema/blob/master/results_schema/schema.py#L79

There could be an interface that doesn't require a SQLAlchemy session, though. Maybe something that takes an engine/connectable, and an experiment hash.

But maybe it should be a soft delete? We can add an is_deleted column to each of the tables, and cascade a soft delete.

_Copied from original issue: dssg/results-schema#5_