Support rotation settings on Element

The Element model will require a few new fields to support image rotation:

• rotation_angle is a positive integer, between 0 and 360, defaults to 0
• mirrored is a boolean, default to False that should only define mirroring on the vertical axis to match IIIF spec

These fields should be editable from:

• the django Admin for Element
• the API endpoints:
  • CreateElement as non required fields
  • UpdateElement as non required fields
  • PartialUpdateElement as non required fields

No rotation is applied by the backend, the data is only stored.

All children elements should be rotated using the same parameters.

No need to support those changes on bulk endpoint CreateElements for now. This may come at a later stage, if needed.

As an aside note, these changes do not require #787 (closed)