Add original_heading_id to investments

Investments can be reclassified to a different heading during the participatory budget process.

Whilst we are recording this change of heading in the `previous_heading_id` attribute, we are only keeping the _last_ heading. If there are multiple reclassifications we lose this chain of reclassifications.

In this commit we are adding an `original_heading_id` attribute, that will only be set once, when creating the investment, and will not get lost with multiple reclassificaitons of an investment.

db/migrate/20190531173316_add_original_heading_id_to_investments.rb

@@ -0,0 +1,5 @@
+class AddOriginalHeadingIdToInvestments < ActiveRecord::Migration[5.0]
+  def change
+    add_column :budget_investments, :original_heading_id, :integer
+  end
+end

db/schema.rb

@@ -294,6 +294,7 @@ ActiveRecord::Schema.define(version: 20190607160900) do
     t.datetime "ignored_flag_at"
     t.integer  "flags_count",                                 default: 0
     t.integer  "tracker_assignments_count"
+    t.integer  "original_heading_id"
     t.index ["administrator_id"], name: "index_budget_investments_on_administrator_id", using: :btree
     t.index ["author_id"], name: "index_budget_investments_on_author_id", using: :btree
     t.index ["community_id"], name: "index_budget_investments_on_community_id", using: :btree