Date filter in iD should prefer EDTF dates over basic dates

[1ec5]

Now that OpenHistoricalMap/iD#187 has added an EDTF date field and validator, the user might expect the date filter to filter features based on the EDTF dates when available. This would make it easier to edit in areas without many certain dates.

The edtf.js library we’re using can return the minimum and maximum possible dates of an EDTF date, optionally with the same precision as the input.

This filtering would be inconsistent with the renderer until we implement similar EDTF-based filtering in the tile generator: #15.