Changed width of commit signature avatar

[NULLx76]

Changed width of commit signature avatar from auto to 16px.
Fixes #12959

I've build and ran gitea locally to confirm that this fix resolves the issue.

[codecov-commenter]

Codecov Report

Merging #12961 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12961      +/-   ##
==========================================
- Coverage   42.65%   42.64%   -0.02%     
==========================================
  Files         669      669              
  Lines       73493    73493              
==========================================
- Hits        31351    31338      -13     
- Misses      37058    37067       +9     
- Partials     5084     5088       +4     

Impacted Files	Coverage Δ
modules/indexer/stats/db.go	43.47% <0.00%> (-17.40%)	⬇️
modules/indexer/stats/queue.go	64.70% <0.00%> (-11.77%)	⬇️
modules/git/repo_language_stats.go	52.83% <0.00%> (-3.78%)	⬇️
modules/git/utils.go	73.77% <0.00%> (-3.28%)	⬇️
modules/log/event.go	57.54% <0.00%> (-1.89%)	⬇️
modules/git/repo.go	46.19% <0.00%> (-0.51%)	⬇️
modules/queue/unique_queue_disk_channel.go	55.38% <0.00%> (+1.53%)	⬆️

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