When the collector is down, dashboard cards and subject tables may seem to be inconsistent

[fniessink]

Describe the bug
When the collector is down, no now measurements are made. After a while the frontend will start to notice this and mark the status of metrics as unknown in the dashboard. However, in the subject table, the last know status of metrics will still be shown (e.g. a metric meeting its target is still green). The measurement value will get a red background and the popup will explain to the user why the measurement value has the red background.

Expected behaviour
When the above scenario happens, mark the metrics as unknown (white) in the subject table. Check that the popup explains that the collector hasn't measured the metric for more than 1 (2?) hours.

CC: @Sebastiaan127001