Refactored folder-filters -- copying Task ids from SearchCacheElement to
FolderTask such that only one type of query is necessary for accessing multiple Folders. (added .cache_searchcache and .cache_tag_ids)
Search now can accept tag IDs instead of names.
Added cache_many_function decorator.
Fixed parse_solution_stats (added solvable task count to user_stats).
Fixed some other permissions / tags and similar bugs.