Use user group icons of standard sizes

[agoujot]

18px is not a standard size but 20px is.

Bug: T419426

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.69%. Comparing base (4b8eb28) to head (82d57fd).
⚠️ Report is 261 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##               main     #623       +/-   ##
=============================================
- Coverage     71.30%   60.69%   -10.62%     
- Complexity     1275     1409      +134     
=============================================
  Files            46       46               
  Lines          3792     4320      +528     
=============================================
- Hits           2704     2622       -82     
- Misses         1088     1698      +610     

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