Humanize reported durations

[fbarl]

Resolves #2875.

Changes

• Collected and briefly documented all the data types we're sending through report fields - except for string which is implicit, those data types were so far: number, datetime, ip.
• Added a new data type duration and sending that data from the backend in a number format instead of formatted string so that the frontend can take of nice formatting.
• Nice formatting of durations on frontend - rounding down the values to be consistent with other fields; unit tested.

Notes

• The changes should be compatible with old probes/reports as durations would be simply sent as string and shown in the old format in that case. Merging reports should also be fine as reconciliation of those fields is simply done by rewritting (needs to be double checked still).

[fbarl]

Scope uptime should be shown in days

[dlespiau]

OK, spent some time in the report structures and the values in the "latest" map are (string, string). So we're actually not changing the type here, we serialise strings.

The merging code for "latest" just picks up the most recent value, no problem there either.

The display logic in the UI is what matters then, but because of the new dataType, the UI will do the right thing.

lgtm!