Support arbitrary extra sensors in the database

[davidrg]

Currently any station supporting more than the basic common sensors provided by the likes of the FineOffset WH1080 require extra database tables and special support added through out desktop app and web UI. This makes supporting extra sensors quite a lot of work.

A way is needed to support arbitrary extra sensors for any station without code changes. This would allow things like one or more Davis AirLink stations to be added to any random existing station. It would also enable the wider selection of sensor configurations allowed by the WeatherLink Live.

Some work towards this has already been done on the configuration side. For Davis stations, extra Leaf/Soil station and extra Temp/Humidity stations are configured via the database with support for renaming and turning on and off individual sensors. Whats missing now is a generic way for adding extra sensors via configuration and a way of storing data for those sensors.

Further work on this really shouldn't be tackled until #41 and probably #42 are dealt with - otherwise its just making that work harder.