Fixed #92: Incremental fix for hiding chatlogs loader.

Template.chat_rows.created and Template.pm_chat_rows.created does not get
fired at all when there are no logs for the selected chat room. So, checking
for 'last_log_id == null' in the created methods was futile. Rather, we
do this check on 'last_log_id' in template helpers 'channelChatLogs',
 'pmChatLogs' and 'serverChatLogs' responsible for setting creating
logs cursor for printing logs and setting Session value for 'last_log_id'.