Move image download to image task

[AWare]

Having images in the front task output was causing step function to fail due to output being too large.

[markjamesbutler]

@AWare I'm not super clear on how this works but I assume we might still have problems if there are lots of images on the Front and that causes the this step to fail. Let's catch-up on Monday. It would be good to know exactly what limit was exceeded and how we can prevent that happening in future.

[AWare]

Sorry for the lack of detail.

Previously, the list of images was extracted from each front and output. This would then be input into the image downloading step of the step function. This caused the output/input to grow beyond allowed step function limits. This change moves the collate images step into the image downloader task so they are never exposed to the step function. This will stop the output growing with size of edition,

[jeanlauliac]

Curious, @AWare how did you detect the problem in production? Alarms from the step function via email?