[#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.

db/migrate/008_add_payment_term_id_to_contracts.rb

@@ -0,0 +1,9 @@
+class AddPaymentTermIdToContracts < ActiveRecord::Migration
+  def self.up
+    add_column :contracts, :payment_term_id, :integer
+  end
+
+  def self.down
+    remove_column :contracts, :payment_term_id
+  end
+end

db/migrate/009_remove_payment_terms_from_contracts.rb

@@ -0,0 +1,9 @@
+class RemovePaymentTermsFromContracts < ActiveRecord::Migration
+  def self.up
+    remove_column :contracts, :payment_terms
+  end
+
+  def self.down
+    add_column :contracts, :payment_terms, :string
+  end
+end

db/migrate/010_populate_payment_terms.rb

@@ -0,0 +1,14 @@
+class PopulatePaymentTerms < ActiveRecord::Migration
+  def self.up
+    [0, 15, 30, 45, 60, 75, 90].each_with_index do |days, index|
+      name = "Net #{days}"
+      unless PaymentTerm.find_by_name(name)
+        PaymentTerm.create!(:name => name, :position => index + 1)
+      end
+    end
+  end
+
+  def self.down
+    # No-op
+  end
+end