add explicit dimensions to navbar avatar

[silverwind]

this prevents a temporary layout change when navigating between pages with cache disabled.

[codecov-io]

Codecov Report

Merging #9986 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #9986      +/-   ##
==========================================
- Coverage   42.26%   42.26%   -0.01%     
==========================================
  Files         610      610              
  Lines       80370    80372       +2     
==========================================
- Hits        33968    33966       -2     
- Misses      42223    42228       +5     
+ Partials     4179     4178       -1

Impacted Files	Coverage Δ
models/list_options.go	57.89% <ø> (ø)	⬆️
routers/org/members.go	0% <0%> (ø)	⬆️
routers/repo/commit.go	29.66% <0%> (-0.26%)	⬇️
models/unit.go	37.03% <0%> (-2.47%)	⬇️

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[silverwind]

Before:

After:

[silverwind]

Also done the same on the frontpage. There is a lot more to be done on other pages, but I'll leave those for later.