expose platform(s) story first found on

[rahulbot]

This came up again with researchers - they want to know which platform a story was first found on in a topic. My old write-up: " We need to be able to mark seed stories (at least) with which source they came from. Perhaps tagging is the easiest solution for this? We could also potentially mark spidered stories with which platform(s) they were encountered on, but that'd be complicated I think. "

The idea I walked away with was that we could create a subtopic or each platform within the topic. Any story found in the first round of discovery on that platform would be added to that subtopic. So if story A was found in Google search results, and also already in our Media Cloud DB, it would show up in the "Google platform" and "Media Cloud platform" subtopics. One subtopic for each platform; stories in each are the ones discovered on that platform; one story can show up in multiple of these "platform" subtopics.

There are other ways to implement this, but the subtopic solution might be lowest friction based on what we already have with URL Sharing subtopics. Thoughts?