Use srcset for author avatar

[danielepolencic]

The current avatar image in a blog post is oversized.

We should change the default image from:

<img src='assets/authors/daniel_weibel.jpg' alt='Daniel Weibel' />

to

<img src='assets/authors/image-64x64.jpg' alt='<name>' srcSet="
examples/images/image-64x64.jpg 1x, 
examples/images/image-128x128.jpg 2x
"/>

You can find more info about srcset here: https://www.smashingmagazine.com/2014/05/responsive-images-done-right-guide-picture-srcset/

Update al author avatars to use srcSet.