Support inserting list element at the top

[adrian5]

Feature

It would be nice to have the option of inserting new items at the beginning of the list, instead of appending them at the end. This is vastly less efficient (the whole list has to be moved in the target document) but would greatly improve usability in some use cases.

Example: I have an image gallery constructed like so:

- label: "Images"
  name: "images"
  widget: "list"
  field:
    label: "Image"
    name: "image"
    widget: "object"
    fields:
      - {label: "File", name: "source", widget: "image"}
      - {label: "Caption", name: "caption", widget: "string" required: false}

If the user adds a new image, it appears at the bottom, so they have to scroll all the way down past the other images.

In my case, I would prefer to list to have newest-first order to begin with, as I cannot currently revert the direction of frontmatter data in my SSG (Hugo). If I wanted to manually maintain a newest-first order, I would have to scroll down, edit the new entry, and drag it all the way back up. Consider doing this each time in a list of 50 items – tedious!

Design consideration

There are two separate parts to this problem that can be solved independently.

1. Presentation: The UI could present the user with a reversed list that inserts items at the front, but keep the oldest-first order in the frontmatter. This might conflict with progressive loading, as the developers have discussed in other topics.

2. Storage: NetlifyCMS could allow the user to define the output (and hence input/read) format of the actual frontmatter. This potentially causes a lot more shifting of frontmatter in each operation, but certainly has strong use cases and doesn't clash with progressive loading (if implemented correctly).

Tangentially related: #54

[tomrutgers]

Related to #1244