Add shared crosshairs to all dashboards

[mrjones-plip]

On our dashboards we have a lot of panels, but don't have the "Shared crosshair" feature enabled:

When comparing two panels on the same dashboard that don't line up vertically in layout, it's not always clear where an interesting part of a one panel is in another panel's time line. Shared crosshair fixes this.

here's a demo of it working on another dashboard - note any panel you hover over shows the cross hair on the other 3:

cross.hair.mp4