Add US states to Riverscapes Context Metadata

[philipbaileynar]

It would be helpful if our CONUS projects had the US States as metadata, allowing for easier searching. If we implement this for Riverscapes Context then this metadata should flow downhill to all dependent projects.

We should implement this metadata twice.

1 - As a plan text, human readable form using the same format as the NHD and WBD attribute. So for a HUC in Colorado and Utah it would be:

US States: co,ut

2 - As a JSON "system metadata" item (that is hidden by default in the Data Exchange, but viewable by users)

{"us_states": ["co", "ut"]}