Proposal: hook on block

Proposal is to add a hook when the firewall blocks a request.

This would allow other extensions to be notified on a block event, and do custom logic/logging/notifications.

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An alternative we considered was to add the hook to shouldThisRequestBeBlocked, to allow the $block to also be altered. However, we decided against this because:

• It would be better to implement custom blocking rules using a more robust mechanism, eg: extending a base event class provided by the firewall extension (#22 (closed)), rather than via hooks
• It would run on every page load, although the impact would probably be negligible