[admin] Add a SolidusAdmin favicon

[elia]

Summary

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[codecov]

Codecov Report

Merging #5387 (64d692f) into nebulab/admin (f4c0dc2) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff               @@
##           nebulab/admin    #5387   +/-   ##
==============================================
  Coverage          88.87%   88.87%           
==============================================
  Files                615      615           
  Lines              14861    14861           
==============================================
  Hits               13208    13208           
  Misses              1653     1653           

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[rainerdema]

💯
Should we backport this one to the alpha branch?

[elia]

💯
Should we backport this one to the alpha branch?

I don't think it's necessary, it's cosmetic change 💅, not a blocking bug 👍