Use a worker+redis-based queue to update stats cache

Refs. #276
Set no parents for any Project

children() method of `/projects/` Directory children is not used when
project list is rendered (ProjectSet is used instead).
There is no need to update (cached) stats for `/projects/` Directory.
Init cache for Directory and Store when it is created
Use redis to register TreeItems as dirty

Add is_dirty() method
Provide is_dirty TreeItem attribute to front-end
Fix refresh_stats when there exist obsolete stores
Add page auto-refresh notice (initially hidden; needs some JS wiring)
Add RQ stats to admin dashboard
Add page auto-refresh notice (JS wiring)
Refresh cached statistic as RQ job

refresh_stats command adds RQ job instead of refreshing