feat(chat): remove layout component and allow styling slots

[bmdalex]

feat(chat): remove layout component and allow styling slots

Description

This PR includes the following changes:

1. Refactors the render function for Chat.Message component to not use Layout component

Layout component does not allow the users to achieve proper styling of slots and it introduces a lot of unnecessary DOM nodes; we are using Slot instead.

2. Reduces the size of the tree for Chat.Message by 50% (from 6 to 3):

• before:
  
• after:
  

3. Adds functionality for targeting all subcomponents of Chat.Message:

• avatar;
• messageBody than contains author, timestamp and content;

4. Docs example:

For showing the users the Chat.Message anatomy and how they can style every slot:

5. Styles (see short demo below):

• for focusing the correct area (messageBody instead of the whole Chat.Message)
• Teams theme styles for focus outline for messageBody;
• Teams theme styles for focus underline for links.
  

[codecov]

Codecov Report

Merging #518 into master will decrease coverage by 0.14%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #518      +/-   ##
==========================================
- Coverage   88.32%   88.17%   -0.15%     
==========================================
  Files          42       42              
  Lines        1456     1455       -1     
  Branches      212      212              
==========================================
- Hits         1286     1283       -3     
- Misses        165      167       +2     
  Partials        5        5

Impacted Files	Coverage Δ
src/components/Chat/ChatMessage.tsx	96.22% <100%> (-0.07%)	⬇️
src/components/Chat/Chat.tsx	91.66% <66.66%> (ø)	⬆️
src/components/Layout/Layout.tsx	94% <0%> (-4%)	⬇️

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[layershifter]

I don't have any other comments, LGTM 👍

[kuzhelov]

please, take a look on the comment related to message body's styles - other than that everything looks fine to me