Add pipeline icon to tab for pipeline editor

[ajbozarth]

Unlike other tabs, prior to this any pipeline editor tab was missing an icon

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[ajbozarth]

Small note: The fix for this on the jupyterlab-2.0 branch will require an update coming in r2 next week (open pr: jupyterlab/jupyterlab/pull/7864) and will have to be address in the final updates to that branch once rc2 is out (that pr will finalize the new icon work and will require a handful of final tweaks on our end)