Reveal Theme.data_source, fix data catalog logic

[rhodges]

Currently both Layer model and Theme model contain a 'source' field and a 'data_source' field.

• Source: a URL to the source Data
• Data_source: Text naming/describing the organization that created or serve the data.

The Theme Admin does not expose 'data_source' field for editing, leading to cases where the "Source:" section of the catalog will be a FQDN link to the source that reads as "None". There also seems to be confusion in the toolbar that should be addressed when there is a Data Source but no 'source' link.