Add replace dashboard action

[aalpern]

The export command+import screen is great for copying dashboards from one instance to another; I use it all the time to copy dashboards out of production into a local instance to experiment on them. But there's no way to update an existing dashboard from a complete definition - you can only import it, which creates a new board with a new URL. Being able to update a complete dashboard definition in place from JSON would be fantastic for that workflow.