SF-3565 Fix error when opening a note in a Lynx insight

[pmachapman]

This fixes a race condition on editor load where in some circumstances Lynx insights were inserted before the notes were added, resulting in an inability for notes to be reformatted (i.e. the removing of the highlight state) when they were enclosed by a lynx insight.

As notes are embed blots, not inline or block blots, they should not contain an inline blot. However, as the note thread may still end up being wrapped by a lynx insight's inline embed if a note is inserted within its bounds either by sync or user action, I implemented a default appendChild function for the note thread embed blot, which will quietly drop the event (as the embed blot cannot render an inline blot inside it anyway).

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This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.18%. Comparing base (7140f65) to head (f441aa6).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3473      +/-   ##
==========================================
- Coverage   82.19%   82.18%   -0.01%     
==========================================
  Files         611      611              
  Lines       36449    36449              
  Branches     6000     6005       +5     
==========================================
- Hits        29959    29957       -2     
- Misses       5609     5611       +2     
  Partials      881      881              

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