[NEW] Support all kind of System Messages

[rafaelks]

@RocketChat/ios

Closes #1167

• Message Snippeted;
• Room Changed Privacy;
• Room Changed Topic;
• Room Changed Announcement;
• Room Changed Description;

Jitsi message support will be added in another PR.

[codecov]

Codecov Report

Merging #2303 into develop will increase coverage by 0.01%.
The diff coverage is 33.33%.

@@             Coverage Diff             @@
##           develop    #2303      +/-   ##
===========================================
+ Coverage    26.09%   26.11%   +0.01%     
===========================================
  Files          438      438              
  Lines        15933    15969      +36     
===========================================
+ Hits          4158     4170      +12     
- Misses       11775    11799      +24

Impacted Files	Coverage Δ
Rocket.Chat/Models/Message/UnmanagedMessage.swift	9.52% <ø> (ø)	⬆️
Rocket.Chat/Models/Message/Message.swift	89.18% <ø> (ø)	⬆️
...cket.Chat/Models/Mapping/MessageModelMapping.swift	83.51% <100%> (+0.37%)	⬆️
...ket.Chat/Extensions/Models/MessageExtensions.swift	41.84% <29.41%> (-3.96%)	⬇️

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