Define generic 'sensor' meters

[x] Define new content type and entities table. Currently use types: temperature, humidity, illumination, occupancy.
[ ] Let API endpoint /meters list also sensors.
[x] Show sensors in client right panel.
[x] Add API endpoint /data for sensors data
[ ] Let client read sensors' data.
[x] Let client display sensors' data in chart with secondary Y axis.
[ ] Convert climate meters to sensors.
[ ] Delete climate meters.
[ ] Remove climate meters' content type and tables.

Gizra / negawatt-server