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
Add option to disable screen security

See WhisperSystems/TextSecure#788
Merge pull request #794 from SamWhited/issue788

Add option to disable screen security
Implement trivial support for SMS signatures
Update preference constant names
Fix badly merged code
Fix preferences
Fix pref. names and add strings