Add a structure system to Repositories

Is your feature request related to a problem? Please describe.
Users managing repositories would like to replicate real world hierarchies with a Repository for each of their organisational units. This gives each unit freedom to setup their repository differently from others.

Describe the solution you'd like

• Add a new RepositoryOrganisationalUnit model
• Add a new view that lets staff manage the structure of their ROUs but moving them between parents
• Add a new view at the press level that displays the ROU structure for that press
• Repositories that do not have an ROU should be listed under an "Other" section

class RepositoryOrganisationalUnit(models.Model):
    repository = models.OnetoOneField(
        Repository,
        on_delete=models.CASCADE,
        related_name='organisational_units'
    )
    parent = models.ForeignKey(
        'self',
        on_delete=models.SET_NULL,
        blank=True,
        null=True,
        related_name='sub_units'
    )

Alternative discussed with @mauromsl: Simply at the parent field onto the Repository model itself.

Start by reviewing the existing Repository model and the repository and press views. Define how the proposed RepositoryOrganisationalUnit relationship or the alternative Repository parent field should support staff-managed nesting. Done means staff can move units between parents, the press view displays the structure, and unassigned repositories appear under “Other”.