Insights: Preview Result Button should open in a new tab

Created by: Joelkw

The new preview button is awesome!

But as a followup to https://github.com/sourcegraph/sourcegraph/pull/29698 – the preview button should open the preview in a new tab. This may not have been specified on the designs directly (I didn't see it), but I believe this was the intention given the icon on the preview button which near-universally implies "new tab" (to me). Also, we want users to "preview" the search – they're going to need to come back to insights, so we shouldn't make them navigate to and fro.