WebSearch: collection tree score as in next.

* Updates how the `score` value from `collection_collection` is used
  adopting next way, the lower the score the higher the position.

* Provides an upgrade recipe to update the `collection_collection`
  relationships. It changes the score setting its value from 0 to the
  number of "son collections".

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
I shouldn't be the case.

Loops cannot be entered via de web interface in master and, if you mess up your DB manually, the problem will pop up before running the upgrade script.