Add distinction between exporting filtered and unfiltered tracks about dive

[subdavis]

Followed up with Matt:

• I misunderstood this issue. He wants to be able to export distinguishing filtered and unfiltered tracks. Filtering is ephemeral, so it only makes sense to do this from within the viewer itself (server doesn't know what the filter is: it has to be told)
• Each confidence pair pertains to an entire track. We should bubble this state up onto the track object, because it's wasted to duplicate across all detections.
• We should filter tracks below the confidence thresh out of the track list.
• Might make sense to persist the thresh for each video on the server as metadata, low priority, no action for now.