Fix 4 stale reads in CreateClassification

Closes #507 (closed)

• Creating an element then immediately creating a classification on it could cause a HTTP 400 (element not found)
• Creating an ML class then immediately creating a classification using it could cause a HTTP 400 (ML class not found)
• Creating the same classification twice with a worker_version could cause a HTTP 500 (unique violation)
• Creating the same manual classification twice could cause a HTTP 500 (unique violation, with a different queryset)

changed the description

assigned to @babadie and unassigned @erwanrouchet

Codecov Report

Merging #1051 into master will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1051   +/-   ##
=======================================
  Coverage   87.89%   87.89%           
=======================================
  Files         235      235           
  Lines       15211    15213    +2     
=======================================
+ Hits        13370    13372    +2     
  Misses       1841     1841           

Impacted Files	Coverage Δ
arkindex/documents/serializers/ml.py	100.00% <100.00%> (ø)

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added 6 commits

• 2f133b23...735b3801 - 5 commits from branch master
• a050d4a1 - Fix 4 stale reads in CreateClassification

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enabled an automatic merge when the pipeline for a050d4a1 succeeds

merged

mentioned in commit 50d94542