Implement ActivityPub actor representations

[raccube]

Closes #907

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2c3e156) 95.22% compared to head (1837d4a) 95.26%.
Report is 312 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1239      +/-   ##
==========================================
+ Coverage   95.22%   95.26%   +0.03%     
==========================================
  Files         166      168       +2     
  Lines        2598     2619      +21     
==========================================
+ Hits         2474     2495      +21     
  Misses        124      124              

Files	Coverage Δ
app/controllers/user_controller.rb	100.00% <100.00%> (ø)
app/serializers/image_serializer.rb	100.00% <100.00%> (ø)
app/serializers/user_serializer.rb	100.00% <100.00%> (ø)

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[raccube]

Superseded by #1071 wrong PR

[nilsding]

???

[ch0ccyra1n]

So far this looks like a good start!