Move report fields to the tileset header

[andres-arana]

Related to https://github.com/GlobalFishingWatch/GFW-Tasks/issues/306#issuecomment-252990466.

We need to change the current reports implementation to take the report fields from the cartodb header (in the file referenced in the description field of the cartodb data) instead of the workspace.

The new schema we are going to use to define that a layer supports reports is the following structure in the cartodb layer header:

{
  reports: {
    regions: {
      EEZ: {
        cartodb_column: "reporting_id", 
        format: "json"
      }
    }
  }

On the other hand, the tileset must have the following data in its header:

{
  reports: {
    regions: {
      EEZ: {},
      MPANT: {},
      // ...
    }
  }