Fix polymorphic errors when viewing organization invites

[jenshenny]

Fixes server error for organization invite avo resource. Also added invitable type to see which kind of invite it is.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.24%. Comparing base (d05f69e) to head (3ff8ec1).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6200   +/-   ##
=======================================
  Coverage   97.24%   97.24%           
=======================================
  Files         476      476           
  Lines        9788     9789    +1     
=======================================
+ Hits         9518     9519    +1     
  Misses        270      270           

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