Add last seen time to device card on the device list

[dajtxx]

It would be nice to see the last seen time for each device on the device list. We don't need a label for it, just something like 5m, 1h, 3d.

Not sure about colour coding due to the variable reporting periods. One thing I'm considering for the broker is to track the average interval between received messages so that could be used in the version that integrates with the broker.

[kebab01]

That would be a useful feature to have in the broker. The colour coding scheme in the manage device screen currently just uses hardcoded times. We could also display average time in the app, which could be another metric used to determine if something is wrong with the device or not.