Prevent errors in console during queue rearranging

[marcustyphoon]

Description

Clicking and dragging a post in your queue to rearrange it (either in the queue "channel" page or in patio) generates a ghost post that will throw errors if our react data getters are called on it. Well, technically that's inaccurate; the thing that's created that would be accurately referred to as a "ghost" post is fine; the post that errors is the "fallback" post. Yes, I read the sortablejs readme; no it didn't make it at all clear to me which element is which. Whatever.

Anyway, this excludes the post in question from triggering onNewPosts handlers.

Resolves #1070.

Testing steps

• Click and drag to rearrange a queued post on https://www.tumblr.com/blog/[blogname]/queue. Confirm that no errors appear in the javascript console.
• Click and drag to rearrange a queued post on tumblr patio. Confirm that no errors appear in the javascript console.