Opt-in modqueue for explicit content filter

We can't enable the content filter on our nearly 5k users discord because users attempting to post NSFW content should be banned rather then told to to try again with a new image (and we would want to also mirror bans for to the forums and the game server), and because our community voted against it last time it was tried out of fear of false positives.

A way for moderators to see/review attempts to post explicit content, as well as either whitelist the content if its a false positive or ban the user who attempted to post is absolutely required.

In its current state the content filter is useless.