Add task to migration subscription to new prices

Run via:
`rails subscriptions:migrate_price OLD_PRICE=pro NEW_PRICE=price_123`

lib/tasks/subscriptions.rake

@@ -0,0 +1,47 @@
+namespace :subscriptions do
+  desc 'Migrate Stripe subscription to new price'
+  task migrate_price: :environment do
+    old_price, new_price = *load_prices
+
+    scope = Stripe::Subscription.list(price: old_price.id)
+    count = scope.data.size
+
+    scope.auto_paging_each.with_index do |subscription, index|
+      item = subscription.items.first
+      Stripe::Subscription.update(
+        subscription.id,
+        items: [{ id: item.id, price: new_price.id }],
+        proration_behavior: 'none'
+      )
+
+      erase_line
+      print "Migrated subscriptions #{index + 1}/#{count}"
+    end
+
+    erase_line
+    puts "Migrating all subscriptions completed."
+  end
+
+  def load_prices
+    old_price = AlaveteliPro::Price.retrieve(ENV['OLD_PRICE']) if ENV['OLD_PRICE']
+    new_price = AlaveteliPro::Price.retrieve(ENV['NEW_PRICE']) if ENV['NEW_PRICE']
+
+    if !old_price
+      puts "ERROR: Can't find OLD_PRICE"
+      exit 1
+    elsif !new_price
+      puts "ERROR: Can't find NEW_PRICE"
+      exit 1
+    elsif old_price.recurring != new_price.recurring
+      puts "ERROR: Price interval and interval_count need to match"
+      exit 1
+    end
+
+    [old_price, new_price]
+  end
+
+  def erase_line
+    # https://en.wikipedia.org/wiki/ANSI_escape_code#Escape_sequences
+    print "\e[1G\e[K"
+  end
+end