feat: more clickable message admin

[jennifer-richards]

Uses the subject instead of the "sent" icon as a link to the message instance. Adds a display_subject to ensure even messages that have no subject can still be clicked.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.39%. Comparing base (4ff4805) to head (b919081).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10307      +/-   ##
==========================================
+ Coverage   88.38%   88.39%   +0.01%     
==========================================
  Files         325      325              
  Lines       43464    43524      +60     
==========================================
+ Hits        38417    38475      +58     
- Misses       5047     5049       +2     

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