Add date/datetime/time range filters, combine active/new filter generation logic

This pull request adds range filters for date, datetime, and time types. It uses the new bootstrap daterangepicker.

I also combined the jinja2 logic for active filters (from models/layout.html) with filters.js. The filters are loaded entirely with from filters.js, because I wanted to avoid duplicating more filter generation logic in models/layout.html.

A demo is available here, but I don't plan to leave this page up for long: http://104.236.24.168:5000/admin/post/?flt0_24=2013-11-21+to+2014-11-21&flt1_36=00%3A00%3A00+to+23%3A59%3A59&flt2_30=2014-01-01+00%3A00%3A00+to+2014-11-21+23%3A59%3A59