Rename activitypub app to social

[mfosterw]

The social app will contain the local representation of activities, and the federation will be handled from a new app.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (87879a5) to head (06ee386).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #323   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           49        49           
  Lines         2017      2017           
=========================================
  Hits          2017      2017           

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