support array of uploaders

Carriewave supports a list of resources as uploaders e.g. ` mount_uploaders :images, SomeUploader `
I need to move several separated uploader attributes to a new attribute defined as a list

```
mount_uploader :icon, IconUploader
mount_uploader :foo_icon, FooIconUploader
mount_uploader :bar_icon, BarIconUploader
mount_uploaders :icons, ImageUploader

...

# want to copy all diff. icons to the new list, something like
icons << icon
icons << foo_icon
icons << bar_icon
```

Start by tracing the existing mount_uploader and mount_uploaders entry points and how copying between records is currently handled. Clarify how multiple existing uploader attributes should be combined into the list attribute, including ordering and empty values; done means the requested array-of-uploaders behavior is defined and covered by the relevant tests.

Written by the indexing model from the issue text.