Web: Add animation when automatically scrolling channel sidebar

Description

When the channel sidebar is automatically scrolled either when changing channels using the keyboard or using the channel switcher (as part of ), that scrolling should be smoothly animated with an ease in and ease out transition to make it less jarring

QA Test Steps

See description

Mana

1

Assignee

Andrew Brown

QA Assignee

None

Reporter

Harrison Healey

Epic Link

None

Fix versions

Mattermost Team

UX

Sprint

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QA Testing Areas

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GitHub Issue

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