gatsby-image <noscript> support

[ghost]

• current behaviour
  When deactivating JavaScript gatsby-image is not replacing the placeholder-image. So the grey-box or traced-SVG stays there.
  Demo (go to Dev-Tools and deactivate JS): https://using-gatsby-image.gatsbyjs.org/

• expected behaviour
  no-script users are getting a normal image (without any effekts/lazy loading). Optimally srcset would be used, otherwise they just get a large image.

[fk]

Hi @zionis137! There's a duplicate of this issue in #3109 (and a PR already, #3122 🎉 ).