Support Slack attachments in response to slash commands

Description

GitHub Help Wanted issue here: https://github.com/mattermost/platform/issues/4238

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We support Slack attachments for incoming webhooks, but not in response to slash commands. To improve compatibility with Slack integrations, we should support them for slash commands as well

QA Test Steps

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Mana

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Assignee

Jason Blais

QA Assignee

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Reporter

Harrison Healey

Epic Link

Fix versions

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Mattermost Team

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Sprint

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QA Testing Areas

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GitHub Issue

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Components

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Configure