Set avatar image dimensions

[da2x]

Set the avatar image dimensions to help browsers avoid layout reflows. Helps page load performance and user experience (less content jank).

Here’s an animated PNG demonstrating the problem this patch solved. (This is Codebase.org’s theme, but the issue is generic.)

[codecov-commenter]

Codecov Report

Merging #12654 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #12654   +/-   ##
=======================================
  Coverage   43.28%   43.28%           
=======================================
  Files         646      646           
  Lines       71625    71625           
=======================================
+ Hits        31002    31006    +4     
+ Misses      35608    35602    -6     
- Partials     5015     5017    +2     

Impacted Files	Coverage Δ
modules/avatar/avatar.go	50.00% <0.00%> (-4.77%)	⬇️
modules/git/utils.go	73.77% <0.00%> (-3.28%)	⬇️
services/pull/check.go	47.69% <0.00%> (-2.31%)	⬇️
modules/queue/workerpool.go	58.77% <0.00%> (-1.23%)	⬇️
models/error.go	34.81% <0.00%> (-0.52%)	⬇️
modules/git/repo.go	49.23% <0.00%> (-0.51%)	⬇️
models/issue_comment.go	53.75% <0.00%> (-0.16%)	⬇️
services/pull/pull.go	41.57% <0.00%> (ø)
services/mirror/mirror.go	19.93% <0.00%> (+1.35%)	⬆️
modules/process/manager.go	75.00% <0.00%> (+2.50%)	⬆️
... and 4 more

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[silverwind]

Note for later: We should probably extract avatar rendering to a helper function, e.g. {{avatar .Owner.Name 290}}.

[lafriks]

🚀