Add stacked Message content variant

[thatblindgeye]

We want to look into allowing message content to be stacked, so that metadata + avatar are on the same line, and the actual message content is below it, as seen in the following screenshot:

Acceptance criteria:

• Add a prop to allow opting into this new structure

• Adjust the markup as necessary to place metadata + avatar in the same container, and message content separate

• Check whether the current behavior where the avatar scrolls with the message content doesn't occur

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Jira Issue: PF-3543