Give audio-white and audio-off-white shadow via CSS

[pixelipo]

Fixes regression #7243

audio-white and audio-off-white should be visible in Talk app when video is disabled (thanks to shadow).

This introduces new class .icon-shadow that can be use on any icon. Other icons that (still) have SVG shadow filters will be migrated to this solution in a subsequent PR, because only this one is critical.

To be reviewed and merged with nextcloud/spreed#501

[codecov]

Codecov Report

Merging #7247 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #7247      +/-   ##
============================================
+ Coverage     50.84%   50.84%   +<.01%     
  Complexity    24548    24548              
============================================
  Files          1585     1585              
  Lines         93804    93804              
  Branches       1354     1354              
============================================
+ Hits          47693    47697       +4     
+ Misses        46111    46107       -4

Impacted Files	Coverage Δ	Complexity Δ
lib/private/Security/CertificateManager.php	91.08% <0%> (-1%)	39% <0%> (ø)
apps/files_trashbin/lib/Trashbin.php	72.28% <0%> (-0.25%)	136% <0%> (ø)
lib/private/Server.php	83.31% <0%> (+0.11%)	126% <0%> (ø)	⬇️
core/js/js.js	63.55% <0%> (+0.56%)	0% <0%> (ø)	⬇️