Add Django-constance for dynamic settings

[krestenlaust]

Resolves #448

This is mostly to test whether adding constance as a dependency is justified

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 81.20%. Comparing base (294159e) to head (284be63).
Report is 9 commits behind head on next.

❗ Current head 284be63 differs from pull request most recent head 448a221

Please upload reports for the commit 448a221 to get more accurate results.

Files	Patch %	Lines
stregsystem/views.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             next     #460       +/-   ##
===========================================
+ Coverage   53.49%   81.20%   +27.70%     
===========================================
  Files          33       33               
  Lines        4621     3112     -1509     
  Branches      274      377      +103     
===========================================
+ Hits         2472     2527       +55     
+ Misses       2097      546     -1551     
+ Partials       52       39       -13     

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