Clean up spline-link creation.

[josiah-wolf-oberholtzer]

• The original link should be the one that is animated in D3.
  • It should receive a reference to the intermediate node.
  • It should receive isSpline=false
• The generated links should be renamed for clarity:
  • entity-key-A-role-[entity-key-B] (going to the intermediate)
  • [entity-key-A]-role-entity-key-B (coming from the intermediate)
• The isPrimary key-value is not necessary then

[josiah-wolf-oberholtzer]

Done in e82bb77.