[IMPROVEMENT] Chat theming performance

[Sameesunkaria]

@RocketChat/ios

Now the theme is cached with the message to avoid repeated work. This allows us to not force update the message every time applyTheme is called.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (feature/integrate-rc-view-controller@e55674f). Click here to learn what that means.
The diff coverage is 87.5%.

@@                           Coverage Diff                           @@
##             feature/integrate-rc-view-controller    #2258   +/-   ##
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  Coverage                                        ?   26.12%           
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  Files                                           ?      430           
  Lines                                           ?    16155           
  Branches                                        ?        0           
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  Hits                                            ?     4221           
  Misses                                          ?    11934           
  Partials                                        ?        0

Impacted Files	Coverage Δ
...t/Views/Chat/New Chat/Cells/BasicMessageCell.swift	0% <0%> (ø)
Rocket.Chat/Managers/MessageTextCacheManager.swift	83.33% <100%> (ø)

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[rafaelks]

Thank you @Sameesunkaria!!!