added `target_message` to messages table. `target_message` is the id of the message that is being reacted to. Also added the index to `target_message` column and this is to optimize how we will be fetching reaction messages for a certain message
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Linear Task: [[ https://linear.app/comm/issue/ENG-2265/modify-messages-table-to-include-targetmessageid-column | ENG-2265 ]]