Create DIP in data model and add model and index support

[luis100]

Related with #454

Create a new DIP class with:

• Id (UUID)
• Description
• Permissions (as in AIP)
• List of related AIPs
• List of related Representations
• List of related Files
• Date of creation and last modification
• Permanent: (flag to avoid deletion)
• External
  • Open: URL
  • Delete: URL callback

DIP will live in a new dip storage container, will have a folder for each DIP with the ID as folder name, and inside a dip.json with the serialization of the above class. It will also have an optional data folder where optional files can be created.

storage/dip/011c45ba-2a2c-4ddc-a82b-a0049a937e60/dip.json
storage/dip/011c45ba-2a2c-4ddc-a82b-a0049a937e60/data/file.jpeg

A new index collection must be created for DIP, and maybe a new IndexedDIP class. Must be able to find every DIP related to an AIP, or a Representation, or a File, based on UUIDs.

[nuno-vieira-deel]

Done since 5bd7072