Dimension Links Migration

[Yian]

We should move all dimension links from column-level to node-level. There are a lot of moving parts, which should be done in roughly this order:

(1) Make sure the backend for node-level dimension links is at parity with column-level:

• Switch up all unit tests from column-level dimension links to node-level. The generated SQL should remain at parity with column-level links. This will require fixing a number of bugs, like making sure that selected fields on a dimension node are filtered to only those that are used.

(2) Convert the user-facing interfaces

• Make the UI compatible with changes:
  • Use node-level dimensions when linking dimensions (i.e., convert /nodes/{name}/columns/{column}/ to use node-level dimension links)
  • Add an option to configure complex dimensions
• Make the python client compatible with changes
  • Add an option to the client to configure complex dimension links
  • Keep the current client dimension link methods but use node-level rather than column-level (i.e., change the backend API that's called)

(3) Database migration

• Write script to migrate column-level dim links to node-level