Fix handling of threaded messages around edits & echoes

[t3chguy]

Fixes element-hq/element-web#21633

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This PR currently has no changelog labels, so will not be included in changelogs.

Add one of: T-Deprecation, T-Enhancement, T-Defect, T-Task to indicate what type of change this is plus X-Breaking-Change if it's a breaking change.

[codecov-commenter]

Codecov Report

Merging #2267 (f5595d2) into develop (d6f1c6c) will increase coverage by 0.35%.
The diff coverage is 90.62%.

@@             Coverage Diff             @@
##           develop    #2267      +/-   ##
===========================================
+ Coverage    58.96%   59.32%   +0.35%     
===========================================
  Files           91       91              
  Lines        16326    16349      +23     
  Branches      3766     3768       +2     
===========================================
+ Hits          9627     9699      +72     
+ Misses        6699     6650      -49     

Impacted Files	Coverage Δ
src/models/event.ts	68.75% <ø> (+1.92%)	⬆️
src/event-mapper.ts	95.65% <66.66%> (-4.35%)	⬇️
src/client.ts	37.57% <85.71%> (-0.58%)	⬇️
src/models/room.ts	57.33% <90.14%> (+2.86%)	⬆️
src/models/event-timeline-set.ts	79.92% <100.00%> (+1.93%)	⬆️
src/models/relations.ts	50.66% <100.00%> (+13.33%)	⬆️
src/models/thread.ts	69.10% <100.00%> (+6.90%)	⬆️
src/sync.ts	65.37% <100.00%> (-0.22%)	⬇️
... and 3 more

[germain-gg]

Looks good at large! I'll give this a go and test it further