Set one web protection setting

Stores one setting in one scope (JSON body: machine, key, value). value "" clears it, so the scope inherits again. An unknown key or a value out of range is a 400 with the reason. autoblock and autoblock.hours are account-wide: setting them on a machine is a 400. Settings that refuse requests (block on, rate.action block, autoblock on) need Raise on your plan, else 403. Machines pick a change up within about 30 seconds. On success replies 204 No Content. Cookie callers must pass CSRF.

This is a write: a read-only API key is refused with 403.