Introduce lock-down mode

[r4zzz4k]

For cases when spammer activity goes out of control, it's useful to have a tool which momentarily limits ability for new uses to make harm to a community. This could include blocking any messages from a newcomers for a limited duration with notification via pinned message, or maybe less restrictive action.

Also we could have a partial lock-down mode, which limits chat activity only when no one is actively watching, e.g. at night time or based on admins / high carma members activity (latter one may not be available via bot API though).