Use the django checks framework to diagnose errors

[kfdm]

Hoping to use to solve parts of #266

[codecov-io]

Codecov Report

Merging #271 into master will decrease coverage by 0.52%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #271      +/-   ##
==========================================
- Coverage   47.22%   46.69%   -0.53%     
==========================================
  Files          57       58       +1     
  Lines        2651     2681      +30     
==========================================
  Hits         1252     1252              
- Misses       1399     1429      +30     

Impacted Files	Coverage Δ
promgen/apps.py	37.50% <0.00%> (ø)
promgen/checks.py	0.00% <0.00%> (ø)

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