Restore predictor-events route, dumps extracted events

[adelavega]

Closes #706

This generalized the logic used in tasks.io to dump PredictorEvents even for ExtractedEvents. It creates PredictorEvents on the fly when necessary. Otherwise, it uses fast dumping for regular PredictorEvents.

Same function is used for dumping events in celery/tasks.

[codecov-io]

Codecov Report

Merging #720 into master will increase coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #720      +/-   ##
=========================================
+ Coverage   85.66%   85.8%   +0.14%     
=========================================
  Files          66      66              
  Lines        2936    2965      +29     
=========================================
+ Hits         2515    2544      +29     
  Misses        421     421

Impacted Files	Coverage Δ
neuroscout/core.py	67.74% <ø> (ø)	⬆️
neuroscout/resources/__init__.py	100% <ø> (ø)	⬆️
neuroscout/resources/predictor.py	88.88% <100%> (+1.21%)	⬆️
neuroscout/tasks/utils/io.py	96.22% <100%> (-1.04%)	⬇️
neuroscout/utils/db.py	93.22% <100%> (+4.64%)	⬆️
neuroscout/schemas/predictor.py	100% <100%> (ø)	⬆️
neuroscout/tests/api/test_predictor.py	100% <100%> (ø)	⬆️

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