Create attachment model and hook it up to todo

An attachment has write permissions on group so that managing
attachments work from different users in same group, i.e. user apache
and user mail.

app/models/todo.rb

@@ -24,6 +24,7 @@ class Todo < ActiveRecord::Base
     :source => :predecessor
   has_many :pending_successors, -> {where('todos.state = ?', 'pending')}, :through => :predecessor_dependencies,
     :source => :successor
+  has_many :attachments, dependent: :destroy
 
   # scopes for states of this todo
   scope :active, -> { where state: 'active' }