Add columns to table and csv download.

[akthom]

I suggest we add two more columns to the samples table under the interactive explorer. These should also be including in any csv downloads.

1. there should be a column called "Source URL" that links to the sample's landing page in its source repository
2. There should also be a column called "PID" that lists the sample's identifier. This already appears in the CSV download, but it should be in the samples table as well.

[rdhyee]

Live on production now (verified fresh-context on isamples.org): the samples table has PID (hover for the full identifier) and Source URL ('record ↗' linking to the sample's landing page) columns, and CSV downloads include a source_url column (appended last, so existing column positions are unchanged). — rbotyee 🤖