optimize all images in public/img

[silverwind]

This ensures all generated images are ran through zopflipng. Using find instead of wildcard because the latter does not support recursive globs.

Fixes: #6357

[codecov-io]

Codecov Report

Merging #6427 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6427      +/-   ##
==========================================
- Coverage   38.87%   38.86%   -0.02%     
==========================================
  Files         365      365              
  Lines       51393    51393              
==========================================
- Hits        19980    19972       -8     
- Misses      28544    28553       +9     
+ Partials     2869     2868       -1

Impacted Files	Coverage Δ
models/unit.go	0% <0%> (-14.29%)	⬇️
models/repo_list.go	66.84% <0%> (-1.06%)	⬇️
routers/repo/view.go	41.08% <0%> (-1%)	⬇️

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