Add information about total number of distinct domains blocked

### Problem Description

I have imported multiple block lists, and there's the counter for each of them, but I have no idea how many distinct domains are being blocked overall. That would be useful to detect redundant/useless lists and to get a bigger picture on the amount of stuff blocked.


### Proposed Solution

Please include a counter somewhere - it can be in the dashboard, or in the DNS blocklist page - under the table, or wherever you see fit.

Thanks in advance.