web: Don't let the refresh countdown display go negative

Only happens when the reload doesn't happen instantly (e.g. with a slow
connection like mine, or when the valen host has problems).
valen: Format JSON output for readability only with the --pretty-json switch
web: Report age timer uses minutes, "a minute ago/less than a minute ago"

There is little point in displaying the age in seconds when it's less
than a minute.