Add references.insert to references collection

Adds the ability to create reference files via an existing claim.

@claudiofullscreen Don't attempt to test this without talking to me first.  Creating reference files in the CMS should not be done without caution.

This adds some shared functionality from Actions::List which should probably be DRY-ed up.

Allows for creation of references **from an existing claim** :

```ruby
content_owner.references.insert(claim_id: 'abcd', content_type: 'audiovisual') #=> #<Yt::Models::Reference
```