Ability to preview a link

[mikewnuk]

When hovering over a link, we should show a tooltip that lets a user preview the link.

This is what Medium does:

Medium doesn't allow editing a link once it's created, but I think we should retain that capability. Maybe add an edit link in the tooltip, too. Clicking the link could still bring up the edit dialog, or we can do away with that completely in favor of the edit button in the tooltip.