timeline: when trying to edit/abort on a stale local echo, retry on matching remote echo

[bnjbvr]

It's quite easy for consumers of the UI crate to get a stale EditInfo or EventTimelineItem, when trying to abort/edit an event:

1. embedder gets an EditInfo or EventTimelineItem from the local echo
2. timeline updates the item to a remote echo
3. editing or aborting using the EditInfo / EventTimelineItem obtained in step 1 would fail, before this patch.

So this is some kind of time-of-check/time-of-use bug, with no security implications.

After this patch, we use better strategies, both for edits and redactions:

• for edits, we try to find an updated local item in the timeline items that would now have an event id (== it's been sent), or a matching remote echo (that does have an event id). This covers all the transitions (local echo => local echo with an event id => remote echo with an event id).
• for redactions, we use the more precise TimelineItem.identifier() method, which will get an event id if the local echo has been sent. There's no TOCTOU issue, because the redact() method takes the event timeline item directly, and not an EditInfo.

Part of #3361.

[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 84.25%. Comparing base (d60ec55) to head (7824e8f).
Report is 12 commits behind head on main.

Files	Patch %	Lines
crates/matrix-sdk-ui/src/timeline/mod.rs	67.56%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3640      +/-   ##
==========================================
+ Coverage   84.23%   84.25%   +0.01%     
==========================================
  Files         259      259              
  Lines       26555    26575      +20     
==========================================
+ Hits        22369    22390      +21     
+ Misses       4186     4185       -1     

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[poljar]

As far as I understood this PR, I think it looks good. Left a small doc nit.