Add the graph editor in the story pages

[Maartenvm]

Image says all...

• Add button to open dialog
• Dialog to edit graph from the page editor.
• Add the Vega Lite editor to an iframe
• Make a short 'tutorial' on the page itself to guide the user to copy URLs and text to the right boxes.

[kretep]

Maybe for later, but to make sure we don't forget: instead of using an iframe, we could use the Monaco editor for editing the vega json. This editor is used in the vega editor (and also in VSCode btw) to edit the json and supports autocompletion out of the box because there is a schema available. The generated chart could easily be updated using the embed function.