[D&D] Ensure relationships to index patterns are saved correctly

[joshuarrrr]

As a follow-up to #1870, we need to make sure we're saving the index pattern as a proper saved object relationship, rather than just an id in VisualizationState.

As an example, the relationship of a basic metric visualization looks like this:

[
  {
    "name": "kibanaSavedObjectMeta.searchSourceJSON.index",
    "type": "index-pattern",
    "id": "d3d7af60-4c81-11e8-b3d7-01146121b73d"
  }
]