feat(images): construct srcset in mixin to ensure they work with additional params

[mjpvs]

The existing srcset generation where we just add ?size=x to the src has some issues if the src already has a query parameter in it which happens in the external preview mode. Juan Luis has sorted most of the cases in the freemarker, this gets the last few in the component library and centralises our srcset generation logic in a mixin to get rid of a bit of code duplication