Avoid using image_block/random as an alias url for an item

[bharat]

It causes a second parallel set of aliases for all items which will cause crawlers to recrawl them. Two possible solutions:

1) use a 301 instead of a 302 to avoid confusion

2) add a param to the url to reset the display context. We can handle that in the framework, ie "?no_ctxt" - not sure if that'll still get crawled separately - probably not.