Implement Horizontal Separator common component

Description

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QA Test Steps

Validate that the appearance of separators in post-list (new message, date) look consistent with previous versions.

Reporter

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Status

Mana

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Assignee

Eli Yukelzon

QA Assignee

Lindy Isherwood

Reporter

Eli Yukelzon

Fix versions

Mattermost Team

Platform

Sprint

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Labels

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QA Testing Areas

Messages

GitHub Issue

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Components

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