[Issue #6414] Delete thumbnails using Django file storage API

[mtnorthcott]

PR to perform thumbnail deletions using the Django file storage API.

Previously, os.remove was used to delete files from Django storage, which did not function correctly for remote storage as this is not implemented. This fix uses Django's storage.delete method instead which will work if the chosen storage plug-in implements it.

Checklist

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[codecov]

Codecov Report

Merging #6415 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6415      +/-   ##
==========================================
+ Coverage   59.00%   59.02%   +0.01%     
==========================================
  Files         184      184              
  Lines       20569    20569              
  Branches     3415     3416       +1     
==========================================
+ Hits        12136    12140       +4     
+ Misses       7360     7356       -4     
  Partials     1073     1073