Change artifacts/preview to prevent the embeds-preview bug

[rorysaur]

Some embeds, such as Figma, mess up the UX of the Markdown input, because every time they are re-rendered in the real-time preview, it takes the focus out of the input box so you can't keep typing.

Option 1:
Instead of real-time preview as you type, make the preview a separate tab that you can click on (similar to what GitHub does with issues).

• I like this one personally, because then you get the full width of the screen for editing and for preview, and we'll probably have to do this for smaller screen sizes anyway. (For smaller screen sizes it would be weird to go 1-column and have the preview below the editor, because you would have to scroll up and down between them.)

Example:

Option 2:
Keep the real-time preview, but don't render embeds in it.

The above is non-exhaustive, feel free to suggest more options.