Polling for notes on images doesn't render new discussions

On a commit or MR with an image, existing discussions will update in real-time, but new discussions won't be added in real-time.

Similar to https://gitlab.com/gitlab-org/gitlab-ce/issues/38677 but for image discussions, not text.

You can see discussion 2 is not on the left window.

Screen_Shot_2017-11-04_at_15.19.58a

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