Improvements and fixes

Production and analysis graphs now mark the periods when snow lay on the ground. The monitor and the production page now show which devices are still reporting and when each was last seen.

• Periods with snow on the ground are now shaded in the production and analysis graphs and named on hover.
• In the monitor, an info button beside a plant's connection status opens a list of its data loggers and how each one is doing.
• An inverter that has sent data in the last 2 hours now shows as reporting in the monitor.
• The inverter table on the production page now shows, for each inverter, whether it is reporting and how long ago it was last seen.
• A plant that is offline, has no data, or is producing above expectation is now shown as such, instead of all three looking inactive.