Start warning about upcoming SECRET_KEY default removal

[gdubicki]

Related to #721

[codecov]

Codecov Report

Base: 85.43% // Head: 85.50% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (f5d29f5) compared to base (9c41b29).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #726      +/-   ##
==========================================
+ Coverage   85.43%   85.50%   +0.06%     
==========================================
  Files          19       19              
  Lines        1078     1083       +5     
==========================================
+ Hits          921      926       +5     
  Misses        157      157              

Impacted Files	Coverage Δ
puppetboard/app.py	83.33% <100.00%> (+0.25%)	⬆️
puppetboard/default_settings.py	100.00% <100.00%> (ø)
puppetboard/docker_settings.py	100.00% <100.00%> (ø)
puppetboard/utils.py	93.18% <100.00%> (+0.24%)	⬆️

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