Add volume_folder to export json

[jcushman]

Add "volume_folder" to exported metadata for volumes, so we can link correctly when there are multiple volumes with the same volume_number.

I also added the missing line reporter = Reporter.objects.filter(short_name_slug=reporter_dir.name).exclude(start_year=None).get(), which I used for the export and had sitting around locally but failed to include in a previous PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.58%. Comparing base (b9abdf8) to head (90768a9).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2208      +/-   ##
===========================================
+ Coverage    63.57%   63.58%   +0.01%     
===========================================
  Files          107      107              
  Lines        11859    11863       +4     
===========================================
+ Hits          7539     7543       +4     
  Misses        4320     4320              

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