Consider most recent upload date as default year in max slider
The year slider is set up now to display the data coverage years as the default minimum and maximum values. This is causing an issue where the slider maximum may be lower than the latest upload year. (e.g. all datasets have coverage before 2014, but there are uploads from 2015, so the slider maximum was 2014). The sliders can still be manually increased, but for users that just click the checkboxes and don't change the sliders, the default search may be exclusive to recent uploads.