1.9 empty outbox warnings

In my fresh Chamilo installation:

If I visit the outbox for the administrator user, I am greeted by 3 undefined variable warnings.

This commits fix those, but it's worth saying that the code in main/messages/ is *kind of* a mess :-)

http://localhost:8000/chamilo-lms/main/messages/outbox.php?f=social
```
Notice: Undefined variable: social_right_content in /Users/diego/Sites/chamilo-lms/main/messages/outbox.php on line 117
Notice: Undefined variable: html in /Users/diego/Sites/chamilo-lms/main/inc/lib/message.lib.php on line 1438
Notice: Undefined variable: social_left_menu in /Users/diego/Sites/chamilo-lms/main/messages/outbox.php on line 156
```