BE - Links to source - metadata (Nathan)

[nam20485]

Add fields to display metadata.

These will be used to display context-sensitive information (data source, links to more information, etc.) about the currently-selected data set in an existing UI element (i.e. legend or dashboard).

This is the same information being compiled in @jaronheard's metadata document: https://docs.google.com/spreadsheets/d/1NdeY6Eda9eJX3ij2lv8JVu-ZnJ_zdub6BJgzlNwSOj0/edit?usp=sharing

and will use the resulting API.