[#4410] Replace Payment Terms with an Enumeration

Instead of using a hard coded value for Payment Terms, they will use
Redmine's Enumeration table.  This provides an admin gui to managing
the value as well as ordering them.

test/unit/contract_test.rb

@@ -3,6 +3,7 @@ require File.dirname(__FILE__) + '/../test_helper'
 class ContractTest < ActiveSupport::TestCase
   should_belong_to :account_executive
   should_belong_to :project
+  should_belong_to :payment_term
   should_have_many :deliverables
 
   should_validate_presence_of :name