The "to_user" and "from_user" relationships name in umessages conflict with other reusable apps 

This is not actually a bug but the the related names "to_user" and "from_user" are often used in reusable apps involving some sort of relations between users. This creates conflicts, for example with django-relationships (see this [issue](https://github.com/coleifer/django-relationships/issues/13)).
I've changed all the occurrences of "to_user" and "from_user" in "um_to_user" and "um_from_user" to better namespace them. Also applied this to the translation files.