Migration: add funnel related tables (#3022)

priv/repo/migrations/20230417095029_init_funnels.exs - restored from:

aa59ade wip
4d039ba Make funnel names unique
4f3216b Unique steps
e55edf8 Rename funnel_goals->funnel_steps
34ceb3f dirty wip

priv/repo/migrations/20230417095029_init_funnels.exs

@@ -0,0 +1,21 @@
+defmodule Plausible.Repo.Migrations.InitFunnels do
+  use Ecto.Migration
+
+  def change do
+    create table(:funnels) do
+      add :name, :string, null: false
+      add :site_id, references(:sites, on_delete: :delete_all), null: false
+      timestamps()
+    end
+
+    create table(:funnel_steps) do
+      add :goal_id, references(:goals, on_delete: :delete_all), null: false
+      add :funnel_id, references(:funnels, on_delete: :delete_all), null: false
+      add :step_order, :integer, null: false
+      timestamps()
+    end
+
+    create unique_index(:funnel_steps, [:goal_id, :funnel_id])
+    create unique_index(:funnels, [:name, :site_id])
+  end
+end