Handle abuse more transparently

Currently we have the ability to a) automatically or b) manually block individual abusers whenever we notice abusive behavior. We're keeping track of a) using logs and b) using an internal Slack channel. Once an abuser is blocked, they are redirected to a page saying "you are blocked".

It would help tremendously to be able to communicate a "reason" to a user. For instance: "network scanning", "mining", "vulnerability scanning", etc. This way we have more influence on actual user behavior, and users are enabled to question our decisions.
Additionally, we could add an audit log to the admin pages which is reviewable, and also contains the timestamp and actor (blocking user). This way we can show proof to users more easily on request, without the need to skim multiple Slack channels for info.

Start by tracing the existing automatic and manual blocking flow, the blocked-user redirect, and the admin pages mentioned in the issue. Define how a blocking reason, timestamp, and actor are recorded and displayed, then verify that blocked users receive the reason and administrators can review the audit history.