Nerf posts that have been flagged as spam

[cellio]

Related to meta:291174

The linked meta post requests a spam reaction to alert readers, but that risks people reacting but not flagging (and then the post never gets addressed). Also, reactions are not anonymous and maybe this should be. An answer suggests disabling links if a post has a spam flag. Let's build on that.

Proposal: if a post has a threshold number of spam flags, disable links. Because that could be confusing, also add a notice along the lines of "links have been removed pending human review" to the bottom of the post. Spam isn't the only problem case here (there's also malware), so I'd rather not say "spam" in the notice, but we can certainly wordsmith this.

As for the threshold: a single flag seems consistent with close votes and our current small size, but I don't know if that will always be true, so let's code it in a way that's easy to change later, or maybe even make it a site setting.

Doing this would make meta:291184 redundant, I believe.