Remove maxcdn from error templates

[Floris272]

Changes

500.html & 503.html used external bootstrap styling for the button, but since CSP is blocking it anyway it has been removed.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.38%. Comparing base (538d661) to head (403f97b).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #675      +/-   ##
==========================================
+ Coverage   82.31%   82.38%   +0.06%     
==========================================
  Files         129      130       +1     
  Lines        2449     2458       +9     
  Branches      199      199              
==========================================
+ Hits         2016     2025       +9     
  Misses        391      391              
  Partials       42       42              

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[Floris272]

will be fixed in #665