Detect COPPA-disabled accounts

There's an undocumented public user account flag that allegedly indicates that it's been disabled/tombstoned due to failing the COPPA gate. This supposedly applies to transient accounts that join servers with 'temporary membership'. If they convert to signing up, and enter an age under 13, they're auto-disabled with the this flag applied. I don't know if this results in a GUILD_MEMBER_REMOVE, but it probably does.