[LDI] Synthesize "is_active" indicator

[amotl]

Within 0c3ec97 and with a minor fixup through fb1301a, we added a feature to synthesize a field called is_active within PostGIS based on the sensor_last_date field coming from the SOS service where we are ingesting sensor data of the IRCELINE sensor network from.

The gist is

ABS(DATE_PART('day', sensor_last_date - now())) <= 7 AS is_active

@d-roet asked for a likewise feature on the LDI network in order to mitigate issues coming from downstream processing of this data, see also [1].

[1] https://source.irceline.be/vaquums/grafana_luftdatenpumpe/issues/27

cc @wetterfrosch