commands: Update refresh_stats to actually refresh stats
pootle_store: Only calculate difference once
pootle_store: Do not calculate target_wordcount, ever

count_words is an expensive operation, and we don't really use
target_wordcount anywhere important.
Where it is actually used can be dropped in later commits.
pootle_store: Update suggestion_count on Suggestion updates
pootle_store: Get store suggestion counts directly from the db
commands: Refresh suggestion count in refresh_stats
pootle_store: Update the cache counts on Unit.delete()
pootle_store/caching: Get rid of old flag_for_deletion() calls
pootle_store: Move stringcount() to where it's locally used
pootle_translationproject: Add wordcount and suggestion_count cache
pootle_translationproject: Use database columns for counts
commands: Update refresh_stats to refresh translation projects
pootle_store: Always update the unit properly if its target has been updated
commands: Update quality checks in refresh_stats