Add fluid-icon

[silverwind]

This provides Firefox (and possible other browsers) with a high-resolution rich icon, in place of the previously removed apple-touch-icon without having to use that one as it lacks an alpha channel. This is the same method GH uses.

https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/UI_considerations#Rich_icons

[codecov-io]

Codecov Report

Merging #10491 into master will increase coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master   #10491      +/-   ##
==========================================
+ Coverage   43.66%    43.7%   +0.04%     
==========================================
  Files         586      586              
  Lines       81408    81411       +3     
==========================================
+ Hits        35549    35583      +34     
+ Misses      41458    41424      -34     
- Partials     4401     4404       +3

Impacted Files	Coverage Δ
models/user.go	49.3% <0%> (-0.13%)	⬇️
models/action.go	48.82% <0%> (-0.59%)	⬇️
services/pull/temp_repo.go	29.05% <0%> (-2.57%)	⬇️
services/pull/patch.go	60.37% <0%> (-2.52%)	⬇️
modules/queue/workerpool.go	56.93% <0%> (-1.07%)	⬇️
models/gpg_key.go	54.81% <0%> (-0.56%)	⬇️
models/pull.go	42.37% <0%> (+0.55%)	⬆️
modules/log/event.go	65.64% <0%> (+1.02%)	⬆️
services/pull/pull.go	36.07% <0%> (+1.17%)	⬆️
modules/queue/unique_queue_disk_channel.go	55.76% <0%> (+1.92%)	⬆️
... and 5 more

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[lunny]

Make L-G-T-M work.