Don't display sender in outgoing message details

Because we don't know it. Fixes WhisperSystems#711.
Don't show letter count inside a push group or when SMS mode is disabled
Merge pull request #805 from wickedshimmy/fix-sender-details

Don't display sender in outgoing message details
Merge pull request #795 from benalbrecht/nocount

Don't show letter count when SMS mode is disabled