The last part of unifying the logic and source of truth for determining eligible messages to be sidebar sources is to have a `canBeSidebarSource` function in each message spec. This diff introduces the constant to each existing message spec, and the next diffs will replace the logic in to `isInvalidSidebarSoruce` to call the message spec. This ensures that we'll only ever need to update the specific message spec in one place and the `ValidRawSidebarSourceMessageInfo` type if we ever want to change this.
Depends on D9324
Addresses 1c of [[https://linear.app/comm/issue/ENG-4849/thread-from-robotext-about-pinned-message-causing-errors#comment-36f2e045 | ENG-4849]].