Add _date_from_milestone_id column

Tracking which milestone the milestone dates are sourced from

db/schema.rb

@@ -955,9 +955,11 @@ ActiveRecord::Schema.define(version: 20180803001726) do
     t.text "description"
     t.text "description_html"
     t.boolean "start_date_is_fixed"
-    t.boolean "due_date_is_fixed"
     t.date "start_date_fixed"
+    t.integer "start_date_sourcing_milestone_id"
+    t.boolean "due_date_is_fixed"
     t.date "due_date_fixed"
+    t.integer "due_date_sourcing_milestone_id"
   end
 
   add_index "epics", ["assignee_id"], name: "index_epics_on_assignee_id", using: :btree

ee/db/migrate/20180711014025_add_date_columns_to_epics.rb

@@ -6,9 +6,11 @@ class AddDateColumnsToEpics < ActiveRecord::Migration
   def change
     change_table :epics do |t|
       t.boolean :start_date_is_fixed
-      t.boolean :due_date_is_fixed
       t.date :start_date_fixed
+      t.references :start_date_sourcing_milestone
+      t.boolean :due_date_is_fixed
       t.date :due_date_fixed
+      t.references :due_date_sourcing_milestone
     end
   end
 end