conversation snippet gross on group update

1. create a group with some friends
2. send some messages between friends
3. notice the "conversation snippet" text in the conversation list view is the last message sent
4. update the group name
5. notice the snippet now reads `<phone #>, <phone #>, etc joined the gr...`

I think it would be better to keep the conversation snippet to message content only, ie: don't update snippet on group update.
