feat: give each user a room-assigned color from Gutenberg's palette

[josephfusco]

Each person keeps the color Gutenberg's getAvatarBorderColor() gives their ID unless someone online already wears it, so the admin bar only disagrees with the editor on a collision until Gutenberg reads the REST color field; the color is only trusted from entry data when it is one of the seven, since clients can write that data and it lands in inline CSS.

Fixes #559

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[codecov]

Codecov Report

❌ Patch coverage is 97.29730% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.98%. Comparing base (5f1ced7) to head (88a0b9e).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #574      +/-   ##
============================================
+ Coverage     95.97%   95.98%   +0.01%     
- Complexity      258      260       +2     
============================================
  Files            25       25              
  Lines          3231     3264      +33     
============================================
+ Hits           3101     3133      +32     
- Misses          130      131       +1     

Flag	Coverage Δ
multisite	95.98% <97.29%> (+0.01%)	⬆️
phpunit	72.54% <97.29%> (+0.24%)	⬆️

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