chore: move mention-user-not-in-channel to afterSaveMessage

[sampaiodiego]

Proposed changes (including videos or screenshots)

Moved the recently added code from #30464 to an afterSaveMessage callback, which doesn't change anything from a feature point of view..

I've also improved the code a little bit removing unnecessary calls to DB.

Issue(s)

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Latest commit: cc05ee8

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[codecov]

Codecov Report

Merging #31263 (cc05ee8) into develop (ca2f4c5) will decrease coverage by 0.02%.
The diff coverage is 33.33%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #31263      +/-   ##
===========================================
- Coverage    49.33%   49.31%   -0.02%     
===========================================
  Files         3299     3299              
  Lines        81080    81079       -1     
  Branches     16554    16553       -1     
===========================================
- Hits         40000    39986      -14     
- Misses       36427    36441      +14     
+ Partials      4653     4652       -1     

Flag	Coverage Δ
e2e	52.62% <ø> (-0.03%)	⬇️
e2e-api	40.66% <33.33%> (-0.02%)	⬇️
unit	76.52% <ø> (ø)

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