Update sensor status per Ajax call

[nhoening]

The status page becomes rather slow when we load all of the states at once, even timing out.

Better to load the status table empty at first and then load status per sensor one by one, and the jobs table in one go.

• new endpoint /sensors/ID/status, with existing logic moved there
• new endpoint /assets/ID/jobs, with existing logic moved there
• make the page load the sensor states one by one (with parallel Ajax calls)
• add a refresh button/link next to each sensor, so users don't have to refresh the whole page
• make the page re-load the jobs regularly from AJAX, as well (each 30 seconds)
• also a refresh link/button for the jobs table as a whole
• display next to each sensor and next to the jobs table how long ago the last update was

[nhoening]

The sensors API would get a new endpoint (/status) with roughly this content:

sensor_statuses = get_statuses(
     sensor=sensor,
     now=now,
)

This is in data/services/sensors/build_sensor_status_data(), which should only collect the information for the rest of the table (and be renamed a bit).

[nhoening]

Branch off from status-page-by-source, please, until that is merged.