Redesign Emoji Picker UI

Description

Emoji Picker does not follow our style guides and could use an overhaul

Some examples:

Note: Consider how we might incorporate a GIF picker as well.

QA Test Steps

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Mana

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Assignee

Unassigned

QA Assignee

None

Reporter

Eric Sethna

Epic Link

Fix versions

Mattermost Team

UX

Sprint

None

Labels

QA Testing Areas

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GitHub Issue

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Components

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