prevent users from adding incidents/FRs to event groups

[srabraham]

groups are intended as placeholders for use by permissions, so users shouldn't be able to add incidents/FRs onto the groups themselves.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.02%. Comparing base (795d7cb) to head (4433d61).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #497      +/-   ##
==========================================
- Coverage   82.46%   82.02%   -0.45%     
==========================================
  Files          50       50              
  Lines        4529     4533       +4     
==========================================
- Hits         3735     3718      -17     
- Misses        450      454       +4     
- Partials      344      361      +17     

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