Sort Media Gallery items from most recent to oldest

A side effect of this commit is that in the old chatview, when the image viewer
is open, seeing the previous attachments requires swiping to the left rather than
to the right (which actually matches the behavior of the new chatview)

Author: lissine0

Monal/Classes/DataLayer.m

@@ -2384,7 +2384,7 @@ -(void) unPinChat:(NSNumber*) accountID andBuddyJid:(NSString*) buddyJid
         return nil;
 
     return [self.db idReadTransaction:^{
-        NSString* query = @"SELECT message_history_id FROM message_history WHERE messageType=? AND account_id=? AND buddy_name=? GROUP BY message ORDER BY message_history_id ASC;";
+        NSString* query = @"SELECT message_history_id FROM message_history WHERE messageType=? AND account_id=? AND buddy_name=? GROUP BY message ORDER BY message_history_id DESC;";
         NSArray* params = @[kMessageTypeFiletransfer, accountID, contact];
 
         NSMutableArray* retval = [NSMutableArray new];