Image optimization

[sergiodxa]

Is your feature request related to a problem? Please describe.
Contentz should use srcset to load optimized images for different devices.

Describe the solution you'd like
An initial solution should use convention over configuration and use the defined image as the max quality and try to load the rest adding @quality to the image file.

Describe alternatives you've considered
Detect images used in the /static and optimize them at build time. Use incremental build to avoid optimizing the same image two times.