Use predefined SessMess json format

EmailService is defining a SessMess[String] json format instead of using
the predefined format in SessMessJsonProtocol. Fix this.
Remove Group existance check

Since a check for {,child,parent} Group membership will always cover an
existance check, the existance check is always unnecessary.
Add isMemberOfParent method
Add ServiceErrors relating to Message Services

Add TitleTooShort, ContentTooShort, ResponseToNothing.
Fix Message, Add insertGroupMessage

Fix Message to properly support post and answers by making the parent
Message Nullable. Add timestamp to Message. Add insertGroupMessage method
to DB.
Add post/response methods to GroupMessageService