add "empty" filter, allow switching to a filter with options, add JS versions

Added an "empty" filter to fix issue #487, a demo is available here: http://104.236.24.168:5001/admin/model1/?flt0_23=0

This required fixing filters.js to load the filter input's options when the filter operation is changed.

Also added versions to the admin/js filenames to prevent loading old cached versions when updates are made.