Quality checks calculating: don't use `groupby()`

If queryset isn't sorted by grouped by field `groupby()` doesn't group
result properly i.e. it could give multiple groups for one field.
Quality checks calculation: don't order query result

Related to #3419
Quality checks calculating: update `unit.mtime` directly

If using `unit.save()` new RQ job added. This is too explicit because
all cached stats are updated after (in the end of the process).
Refreshing stats: avoid errors in RQ stats

Avoid errors in RQ stats jobs after quality checks is updated in disabled projects.

- Improve registering cache as dirty;
- Check if project or translation project are disabled in `can_be_updated()`
  method for a Store and Directory.

Related to #3424.