Update Talks

Move localized talks-related strings from <Users.main> category to <Users.talks>.
Update migration: Add triggers to DB for automatically updating some data about every Talk.
Add notification of unread conversations into site header.
Set <emptyText> option of CListView used in messages list and conversations list.
Add <success_alert> block to the talk's page.
Set selection order to "last_message_time DESC" during retrieving user's conversations.
Fix indentation in <init_photos> migration