Fix 404 page for anonymous user

[hartwork]

Meant to fix #624.

The anonymous user has ID None. Previously, this exception was raised:

NoReverseMatch: Reverse for 'user-profile' with arguments '(None,)' not found. 1 pattern(s) tried: ['user/(?P<pk>\\d+)/profile/$']

[codecov]

Codecov Report

Merging #625 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #625      +/-   ##
==========================================
+ Coverage   80.98%   81.03%   +0.05%     
==========================================
  Files          45       46       +1     
  Lines        2918     2927       +9     
  Branches      177      177              
==========================================
+ Hits         2363     2372       +9     
  Misses        497      497              
  Partials       58       58

Impacted Files	Coverage Δ
cabot/templates/base.html	94.87% <100%> (+0.27%)	⬆️
cabot/templates/404.html	100% <0%> (ø)

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[frankh]

Thanks 👍