Emoji-code auto-complete

Description

When colon at start of input, or preceded by a space, offer auto-complete options for emoji

From: https://github.com/mattermost/platform/issues/1071

Example of UI attached. Should offer up/down arrows to navigate, ENTER to select

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Reporter

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Mana

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Assignee

Ian Tien

QA Assignee

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Reporter

Ian Tien

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Mattermost Team

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Sprint

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