console: add a relationships tab in the custom types page

Currently, the only way to create relationships between object types and tables is by creating it from the action's page (`/actions/manage//relationships`). This makes one feel that the `Relationship` is a property of an action whereas it is actually a property of that action's returning type.

At `/actions/types/manage`, we allow users to view their custom types at a single place. We should have a `Relationships` tab on that page so that people can view and manage the relationships of all the types at a single place.