Switch to storing To addresses as a slice

- Changes on-disk storage format
- Changes JSON API
- To and From values are now parsed/formatted by Go's mail.ParseAddress
  function
- Fixed bug in list-entry-template, was not escaping HTML characters
- Updated tests

rest/testmocks_test.go

@@ -70,9 +70,9 @@ func (m *MockMessage) From() string {
 	return args.String(0)
 }
 
-func (m *MockMessage) To() string {
+func (m *MockMessage) To() []string {
 	args := m.Called()
-	return args.String(0)
+	return args.Get(0).([]string)
 }
 
 func (m *MockMessage) Date() time.Time {