[EPIC] Single Metadata backend of DCAT-US3.0

# User Story

In order to simplify data.gov's harvesting and backend, data.gov wants all metadata stored in the DCAT-US3.0 metadata standard in the backend.

## Details

This should remove a number of complex components, including:

- Removing the MDTranslator service for translating between multiple metadata types.
- Simplifying on the FGDC defined metadata crosswalk from ISO to DCAT-US3.0
- Removing complex code logic for DCAT-US1.1 and mapping between 1.1 and 3.0
- Allowing for more complex/robust UI additions focused on 3.0 features.

## When is this done

When all metadata on the backend of catalog.data.gov is in DCAT-US3.0 standard.