fix(Accessibility): Remove role="presentation" from chatMessageBehavior and FocusZone

[sophieH29]

After testing chat with multiple screen readers, we figured out that no role should be assigned to Chat message. Since FocusZone sets role=presentation to container by default, I override it in chatMessageBehavior by setting role=undefined

[codecov]

Codecov Report

Merging #530 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #530   +/-   ##
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  Coverage   88.17%   88.17%           
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  Files          42       42           
  Lines        1455     1455           
  Branches      212      187   -25     
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  Hits         1283     1283           
  Misses        167      167           
  Partials        5        5

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[kuzhelov]

shouldn't it be removed altogether then?

[sophieH29]

that's a good question, I was also considering that.
@jurokapsiar, do you think this role=presentation to be set by default in FocusZone? We always can add in behavior in cases where we need it

[sophieH29]

After discussion with @jurokapsiar, we decided to remove that attribute from FocusZone. Thank you Roman, for raising this question, it always good to have someone to look at the situation with a fresh eye :)
So this attribute is added in the original FocusZone in fabric-ui, and they have some reasons for that, but we don't see the need for it right now, as in most cases we control role in behaviors.
Will update the PR