Option to mark blocklists as "Trusted/not trusted"

As requested, splitting feature request on its own https://github.com/AdguardTeam/AdGuardHome/issues/2102#issuecomment-732330183
### Problem

With the addition of DNS rewrite syntax in upcoming version, it allows syncing rewrites across multiple AGH instance with ease by hosting the list in a web server.

This also opens up the possibility of abuse by malicious actor from one of the lists used by users by redirecting domains to phishing servers using DNS rewrite rules.

### Solution
During the process of adding a block list, a checkbox could be provided that lets user choose whether to import any DNS rewrites from the list or not. This way user could let AGH continue importing rewrites from the list they maintain or trust.

We could also leave the checkbox to import unticked by default for additional security. Also, a brief explanation underneath would help as well. Something along the lines of DNS Rewrite allows overriding DNS records with list specified entries. Only enable this for blocklists where you trust the maintainers.