/ should be a redirect (and not 404)
Show seconds up to five minutes

When setting a limit of 1m30s (which would make sense for a critical
heartbeat with one minute interval) it will turn up in the UI as
'1m' and not '1m30s'.

The new arbitrary limit will be five minutes.
Warnings after errors don't make sense

If a warning heartbeat has a value that exceeds the error value
then just use the error value and disregard the warning one.