Add a full-dashboard JSON editor

This commit embeds the Ace editor into a Ext window, so that users can edit the JSON that defines the whole dashboard all at once.

This is extremely useful for creating dashboards en masse. For example, if you want to replicate a dashboard across two environments, all you have to do is copy the JSON from the first dashboard, find and replace all of the environment names, and copy into a new dashboard. There is no easy way to perform this operation without this editor.