Bugfix: Return the right blank data in _get_checks()

Otherwise we might get a traceback in
pootle_store.views.py:get_qualitycheck_stats.
refresh_stats: Add some additional prints to report progress
refresh_stats: Marginal performance improvement on large projects
Stats: Add fields to cache last created unit and last submission

This will improve performance when retrieving the stats table.
Stats: Update last created unit

Current approach will increase load when adding whole new stores,
translation projects and projects.
Stats: Update last submission
refresh_stats: Update last unit and last submission fields
Stats: Update and retrieve cached unit and submission