Serve /favicon.ico resembling current arachnys/img/icon_*x*.png files

[hartwork]

One of the effects is that you don't get this Django error e-mail any more:

[Django] ERROR: "GET /favicon.ico HTTP/1.0" 500 106

.ico was auto-created as following:

$ ( cd cabot/static/arachnys/img && convert icon_*x*.png favicon.ico && file favicon.ico )
favicon.ico: MS Windows icon resource - 2 icons, 48x48, 32 bits/pixel, 96x96, 32 bits/pixel

[codecov[bot]]

Codecov Report

Merging #615 into master will increase coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #615      +/-   ##
==========================================
+ Coverage   80.98%   80.99%   +0.01%     
==========================================
  Files          45       45              
  Lines        2918     2920       +2     
  Branches      177      177              
==========================================
+ Hits         2363     2365       +2     
  Misses        497      497              
  Partials       58       58

Impacted Files	Coverage Δ
cabot/urls.py	83.33% <100%> (+0.83%)	:arrow_up:

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[hartwork]

Any concerns or thoughts about this pull request?

[hartwork]

@frankh @dbuxton do you have a minute?

[dbuxton]

maybe it's overkill but we should really have a test to confirm that favicon is served with status code 200. Could you add that?

[hartwork]

Added a simple test and rebased against current master. I refrained from using reverse as this use is a lot more clear in my eyes (and the path won't change).

[hartwork]

Tests fixed now, please have a second look.