Turn pump off when it gets cold after 5pm

Turn pump off when it gets cold after 5pm

There's still some water flow through the roof even when the bypass valve
is open. So stop pumping completely if it stays cold for more than half
an hour. But *only* after 5pm, because the pump *does* need to run for
enough hours each day to actually clean the water, and because this way
I don't need to bother with a trigger for actually turning it back on
again — if it's already cold after 5pm, it's unlikely to warm up again.

Bump the timed switch off to 8pm, because this should now be the primary
way the pump is turned off.