Allow AuditlogHistoryField to block cascading deletes

[kbussell]

Fixes #108

Thanks to @andrewwatts for suggesting and researching the approach.

[codecov-io]

Codecov Report

Merging #172 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
+ Coverage   83.26%   83.43%   +0.16%     
==========================================
  Files          19       19              
  Lines         508      513       +5     
==========================================
+ Hits          423      428       +5     
  Misses         85       85

Impacted Files	Coverage Δ
src/auditlog/models.py	83.42% <100%> (+0.47%)	⬆️

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[audiolion]

Wow awesome PR, I have nothing to even say here so I am gonna shut up and merge 👍