WebSearch: improvements for collection admin

• Fix collection loop check in webcoll (in get_ancestors and get_descendants) that was broken for some cases.

• New fix_collection_scores() function in websearchadminlib.py to correct and uniformize all collection scores in collection_collection table.

• Improve speed of perform_checkcollectionstatus by correctly exploiting restricted_collections cache and by checking only if collections have sons without calculating the whole descendants set.