Switch back to open_in_new icon and remove StackBlitz icon

[andrewseguin]

The Stackblitz icon replaced the example's open_in_new icon. This icon is always bright blue and does not match the site theme (any of the themes), in fact it stands out as almost a "primary" button.

[Splaktar]

It matches the StackBlitz branding and doesn't seem to clash with any of the 4 themes supported on the site.

I'll post a PR to have it use currentColor.

[jelbourn]

I specially want to use the standard Material Design icon for open_in_new, which more clearly communicates what will happen. While the implementation is tied to StackBlitz now, the "open externally" idea is independent of where you're going.