Add granblue_id column to WeaponKeys table

This should have a `NOT NULL` constraint, but due to existing data it is nullable until we can update all environments with values.

db/migrate/20230315095656_add_granblue_id_to_weapon_keys.rb

@@ -0,0 +1,6 @@
+class AddGranblueIdToWeaponKeys < ActiveRecord::Migration[7.0]
+  def change
+    # This needs to be NOT NULL, but initially it will be nullable until we migrate data
+    add_column :weapon_keys, :granblue_id, :integer, unique: true, null: true
+  end
+end