feat: add a network-wide presence summary table - #332

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props joefusco.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

▶ Preview in WordPress Playground

Boots a fresh WordPress with this PR's presence-api build, seeds 5 demo users, and drops you on the dashboard.

_{Stress-test variant: 40 demo users · Built from 6173f4202b9a94eca61a31a9ae07e176460551b0. Auto-updates when you push.}

Codecov Report

❌ Patch coverage is 91.11111% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.99%. Comparing base (c222335) to head (6173f42).

@@             Coverage Diff              @@
##               main     #332      +/-   ##
============================================
- Coverage     97.13%   96.99%   -0.14%     
  Complexity      190      190              
============================================
  Files            14       15       +1     
  Lines          2060     2099      +39     
============================================
+ Hits           2001     2036      +35     
- Misses           59       63       +4     

Review findings from #299 that land here are #326, closed by this one, and #330, which is not.