Add 'suspend notes' for antispam team

By request of @azurelunatic: this will allow the SEO spammer suspend squad of the antispam team to leave notes on particular accounts that are borderline or frequently reported, so we don't have to keep lists of "we decided that account was legit, even if it's high-volume" to give to new people.
Suggest using `$u->clear_prop` instead; more convenient.