Show the amount of individual voters in the budgets component

**Is your feature request related to a problem? Please describe.**

Right now, we can see the amount of confirmed and pending votes in the budgets component. As of PR-7010 we can now also see the totals which is great.

However, now that we have multiple budgets where the user can possibly vote (if e.g. vote in all is enabled), these numbers do not indicate the number of individual people who have voted in one or multiple budgets.

**Describe the solution you'd like**

On the budgets index page, I'd like to add two new numbers to the statistics at the bottom of the view:

1. Users with finished votes: X
2. Users with pending votes: X

This would give the admins a better view of the overall progress.

These numbers should only display in the budgets component's admin index view because within individual budgets, these numbers would match the finished and pending votes already visible in those views.

**Describe alternatives you've considered**

Right now we are manually fetching these numbers from the database due to customer requests. This is not very inconvenient.

**Additional context**

Screenshot attached.

**Does this issue could impact on users private data?**

No, it exposes generalized numbers without any private data.

**Funded by**

Mainio Tech is prepared to fund the development of this feature with our own resources.