feat(Chat): add content and renderContent props

[levithomason]

This PR adds the content prop and renderContent callback prop to all Chat components.

Also, the ChatMessage was hardcoded to show/hide certain slots based on if the message is mine. This logic has been moved to the Teams theme styles instead.

[codecov]

Codecov Report

Merging #348 into master will increase coverage by <.01%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #348      +/-   ##
==========================================
+ Coverage   89.73%   89.73%   +<.01%     
==========================================
  Files          64       64              
  Lines        1237     1247      +10     
  Branches      157      181      +24     
==========================================
+ Hits         1110     1119       +9     
  Misses        125      125              
- Partials        2        3       +1

Impacted Files	Coverage Δ
src/components/Chat/ChatMessage.tsx	96.22% <100%> (+0.22%)	⬆️
src/components/Chat/ChatItem.tsx	95.65% <90.9%> (-4.35%)	⬇️

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