Audit the yellowlist for entries that can be removed

[cowlicks]

If a domain has been added to a mdfp group, we might be able to remove it from the yellowlist.

[ghostwords]

Removing domains from the yellowlist is inadvisable as long as pre-2017.7.24 releases are still around (#1474 (comment)).

This depends on #1474.

[cowlicks]

When we upgrade the yellowlist format (to json, or a signed thing) we should make sure to include a safe way to propagate changes to older versions. While still being able to remove things.

[redranamber]

Related to this, I noticed zergnet.com and its subdomains are on the yellowlist and was trying to find a reason why. I can't see any reason why they can't be blocked and checking the logs on github didn't give any info either. So here are probably some other things other than just mdfps that can be removed too..

[ghostwords]

Yeah, zergnet.com is in the original "example whitelist" from 2013 contributed by @jonathanmayer. I don't see documentation around those domains.

[ghostwords]

Yandex domains probably belong in the MDFP list, not the yellowlist (at least not all yandex.com subdomains ...): #649.