[#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.

app/models/payment_term.rb

@@ -0,0 +1,19 @@
+class PaymentTerm < Enumeration
+  unloadable
+
+  has_many :contracts, :foreign_key => 'payment_term_id'
+
+  OptionName = :enumeration_payment_term
+  
+  def option_name
+    OptionName
+  end
+
+  def objects_count
+    contracts.count
+  end
+
+  def transfer_relations(to)
+    contracts.update_all("payment_term_id = #{to.id}")
+  end
+end