Heard & Gated statistics: 6 months of history accessible

The "who heard who" statistics on the info pages of APRS igates and digipeaters now have a pull-down selection for the past 6 months of data. The same selector was also added for the gated and heard maps. The feature had been requested by quite a few users in the past, and it's been on my mind from the moment I added these statistics in the first place. The data was there in the database, I just hadn't gotten around to adding the user interface for jumping between months before now.