Both multimedia and reaction messages were calling the `messagePreview` function. To keep the message specs more modular, we should move `messagePreview` into individual message specs. This was suggested by Ashoat in D5689
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Linear Task: [[ https://linear.app/comm/issue/ENG-2393/refactor-messagepreview-text-function-into-message-specs-for-reactions | ENG-2393 ]]