Generate Srcset / Sizes Attributes for Responsive Images

Images are currently just plain img tags ie
``

For full responsiveness, all images should be utilizing srcset and sizes attributes to do responsive image scaling so it loads the appropriate resolution image for the size of space it will be occupying. That also impacts page speed, since the browser no longer needs to download a higher resolution image than necessary for the fetching screen size.

```

```

I believe the C4SJ site uses jekyll, which has a plugin to autogenerate the necessary markup:
[https://github.com/wildlyinaccurate/jekyll-responsive-image](https://github.com/wildlyinaccurate/jekyll-responsive-image)

Tutorial to implement here:
[https://medium.com/ivo-valchev/jekyll-responsive-images-with-srcset-5da131415d0f](https://medium.com/ivo-valchev/jekyll-responsive-images-with-srcset-5da131415d0f)