Add hierarchy to CRS subjects

[dvogel]

The hierarchy is (at least partially) exposed on both Thomas and congress.gov.

[dvogel]

First pass at this will be constrained to the 2-level hierarchy exposed in the subects list and leave the terms out of the hierarchy.

Later we can try to use a clustering algorithm to infer a hierarchy for the terms.

[drinks]

:thumbsup:

[dvogel]

Fixed in c2ded2b9f78e635739638468da79f461b1f410f8