Add ordering to redirect manager in netlify

[adamhughes86]

A nice to have would be for the JSON and/or Netlify CMS to order the redirects alphabetically. If the list gets long, it might be hard to see if a redirect already exists (although if it does, you probably wouldn't be trying to add it). The list is sortable via drag and drop, but that's reliant on user intervention. As far as I can see, the list widget doesn't support this natively

Originally posted by @yndajas in #1055 (review)