Change jitp>product relation to has_many

Actual relation between jitp and product is has_one. There is no valid
reason for limiting JITP enabled certificate to one per product.

This commit:
* updates Ecto schema
* adds migration to remove unique_index on jitp.product_id

Fixes nerves-hub#871

apps/nerves_hub_web_core/lib/nerves_hub_web_core/products/product.ex

@@ -19,7 +19,7 @@ defmodule NervesHubWebCore.Products.Product do
     has_many(:firmwares, Firmware)
     has_many(:product_users, ProductUser)
     has_many(:users, through: [:product_users, :user])
-    has_one(:jitp, CACertificate.JITP)
+    has_many(:jitp, CACertificate.JITP)
 
     belongs_to(:org, Org, where: [deleted_at: nil])
 

apps/nerves_hub_web_core/priv/repo/migrations/20221220101029_remove_jitp_product_uniqueness.exs

@@ -0,0 +1,7 @@
+defmodule NervesHubWebCore.Repo.Migrations.RemoveJitpProductUniqueness do
+  use Ecto.Migration
+
+  def change do
+    drop unique_index(:jitp, [:product_id])
+  end
+end