arc_ecto: Create changeset step to auto-delete images

[eteubert]

When an entity with an ecto attribute is updated or deleted, the attached image must be deleted as well. There must be no images stored that are not referenced by any entity.

It should be implemented as a generic changeset function that can be reused by all entities. Pseudo code:

  def changeset(entity, attrs) do
    entity
    |> cast(attrs, [...])
    |> maybe_delete_existing_attachments(attrs, [:image])
    |> cast_attachments(attrs, [:image], allow_paths: true, allow_urls: true)
  end

Requirements:

• remove image when deleting entity
• remove previous image when updating entity with new image
• do not remove image if changeset is invalid

See also:

• "One does not simply delete an Arc attachment" One does not simply delete an Arc attachment stavro/arc_ecto#40