Hackdna/filestoreitem fix

[hackdna]

Solve #2380, improve error handling during data set import and cleanup FileStoreItem management using Django admin site.
Contains a DB migration for the file_store app.

[codecov-io]

Codecov Report

Merging #2386 into develop will decrease coverage by 0.01%.
The diff coverage is 38.88%.

@@            Coverage Diff             @@
##           develop   #2386      +/-   ##
==========================================
- Coverage    50.62%   50.6%   -0.02%     
==========================================
  Files          383     383              
  Lines        26525   26532       +7     
  Branches      1241    1241              
==========================================
- Hits         13427   13426       -1     
- Misses       13098   13106       +8

Impacted Files	Coverage Δ
refinery/data_set_manager/views.py	43.29% <0%> (-0.1%)	⬇️
refinery/file_store/tasks.py	43.38% <33.33%> (-0.77%)	⬇️
refinery/file_store/models.py	61.11% <37.5%> (-0.87%)	⬇️
refinery/file_store/admin.py	82.35% <75%> (-5.15%)	⬇️

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