Performance issues with hide completed on dashboard

### Describe the problem

When user translations list is too long (for example when there are thousands of components within a watched project like on https://weblate.eso-spolszczenie.eu/projects/eso-spolszczenie/), hiding completed translations takes seconds:


The problematic code is here:

https://github.com/WeblateOrg/weblate/blob/e39c9e8e988905d0c2083777feeb51e1f128d3df/weblate/trans/views/dashboard.py#L241-L255

The `prefetch_stats` causes the complete queryset to materialize, which fetches thousands of translations and components from the database. The queries are fast to perform, but parsing results takes 2 seconds in this particular case. We throw away most of these results later because we paginate the results in the end when rendering the template.

### Describe the solution you'd like

`get_untranslated` could lazily prefetch stats for chunks of the queryset as it goes through the queryset

### Describe alternatives you've considered

Other options might be:

* Incorporating is translated flag in the database so that this filter can be done there instead of doing that in Python
* Dropping support for hiding completed translations on the dashboard, it was originally requested in #677

### Screenshots

_No response_

### Additional context

@tomkolp This is why the dashboard on your instance is slow for most of the users.