Use less-ugly gravatar icons already used in OBS and progress

[okurz]

Related progress issue: https://progress.opensuse.org/issues/30124

[okurz]

@lnussel you like this more like in https://progress.opensuse.org/issues/30124#note-10 ?

[codecov]

Codecov Report

Merging #1822 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1822      +/-   ##
==========================================
+ Coverage    90.3%   90.31%   +0.01%     
==========================================
  Files         139      139              
  Lines        9978     9978              
==========================================
+ Hits         9011     9012       +1     
+ Misses        967      966       -1

Impacted Files	Coverage Δ
lib/OpenQA/Schema/Result/Users.pm	95.83% <100%> (ø)	⬆️
lib/OpenQA.pm	69.23% <0%> (-2.57%)	⬇️
lib/OpenQA/Worker/Commands.pm	88.46% <0%> (+2.56%)	⬆️

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