Make satellites data available to users

[153957]

The datastore now stores satellite data, which is data reported approximately every hour by stations which run the latest station-software. This contains the min/mean/max values for the number of satellites and signal strength over the past interval. The intervals are slightly shorter than an hour to make the slices a little bit offset each day (24h cycle of the GPS satellites). This data is stored in the satellites table.

This data could be made available as histograms on the daily summaries or config pages. Or simply as downloadable data via the data form.

(low priority)

[153957]

See #160 for inspiration