fix(message-parser): Italics for channel and user mentions

[yash-rajpal]

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Italics rule doesn't work for some cases like _italics@test_ and _italics#test_ because as soon as it checks @ or # symbol, it tries to match it with user and channel mentions, and the last underscore is matched under UTF8NamesValidation which doesn't makes sense as first part of user and channel mentions rule doesn't parse mentions, so shouldn't use UTF8NamesValidation.

Issue(s)

Further comments

SUP-343

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