Add Registrant API contact update action

Closes #849

app/models/action.rb

@@ -0,0 +1,17 @@
+class Action < ActiveRecord::Base
+  belongs_to :user
+  belongs_to :contact
+
+  validates :operation, inclusion: { in: proc { |action| action.class.valid_operations } }
+
+  class << self
+    def valid_operations
+      %w[update]
+    end
+  end
+
+  def notification_key
+    raise 'Action object is missing' unless contact
+    "contact_#{operation}".to_sym
+  end
+end