Studio - Add image_array and custom array- type fields to sections

Somewhere in-between a field and a list, the main use of this is for mapping datasource fields of type image array.

It should allow the user to add as many images as they like in the normal field UI, and store them as an array of image paths. Like single image fields, it should allow the image-picker component.

We should also support custom_arrays, mainly to help people map an array of image alt tags to complement their image array. In the normal UI, this should allow free-text of strings to be added.