[FEATURE] Show Retina optimized images

This feature must be enabled by adding the second static template
to the configuration. After that, the device pixel ratios of 1.5 and
2.0 are configured via TS - but can be overridden within the CE.

On the client side, the whole thing triggers only if the device pixel
ratio is higher than 1.0 higher or equal than the lowest of the
defined pixel ratios.

If possible, the client selects a better image, fitting it's high-res
screen and displays that.