Hide git and GitHub tab avatar alt text when offline

[smashwilson]

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Description of the Change

Add CSS to hide the alt text for the avatar image in the Git tab when the network is not available.

Same problem as #1528, same solution as #2066. Thanks again @rkbhochalya 😄

Screenshot/Gif

Alternate Designs

I looked into a few other ways to provide custom fallback images so we could use a local "blank avatar" image when applicable. Too much effort for too little benefit, though.

Benefits

The git tab header's layout won't break when the alt text overflows out of the avatar image's bounds.

Possible Drawbacks

None.

Applicable Issues

Fixes #2410.

Release Notes

• The alt text on your avatar in the Git tab no longer overflows when you're offline.

[codecov]

Codecov Report

Merging #2425 into master will decrease coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2425      +/-   ##
==========================================
- Coverage   93.25%   93.07%   -0.18%     
==========================================
  Files         235      235              
  Lines       13163    13163              
  Branches     1889     1889              
==========================================
- Hits        12275    12252      -23     
- Misses        888      911      +23     

Impacted Files	Coverage Δ
lib/shared/keytar-strategy.js	38.75% <0.00%> (-8.53%)	⬇️
lib/models/file-system-change-observer.js	88.00% <0.00%> (-8.00%)	⬇️
lib/git-temp-dir.js	96.77% <0.00%> (-3.23%)	⬇️
lib/git-prompt-server.js	97.29% <0.00%> (-2.71%)	⬇️
lib/helpers.js	86.41% <0.00%> (-1.24%)	⬇️
lib/git-shell-out-strategy.js	99.23% <0.00%> (-0.58%)	⬇️

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