Add EDTF-formatted Start Date and End Date fields to iD presets

[1ec5]

As of OpenHistoricalMap/iD#145, every preset comes with a Start Date and End Date field. We should similarly add fields for start_date:edtf and end_date:edtf to the same presets.

The start_date:edtf field should be a moreField of each preset so that it doesn’t clutter up the sidebar when not in use. It should have start_date as its prerequisite so that it slides in underneath that field. Same goes for end_date:edtf.

[1ec5]

To keep things simple at first, we can define these fields as ordinary text fields. Just as OpenHistoricalMap/iD#138 validates the syntax and suggests the correct ISO 8601 syntax if it detects certain plain-language patterns (mostly English), we can use edtf.js (or generate something similar using EDTF-Parser) to validate the tag, then use edtfy to suggest an EDTF equivalent to any plain-language tag that we encounter. Note that edtfy only supports English and French at the moment. For bonus points, edtf.js can evaluate EDTF as a date range, which would be useful for the date range filter.

Eventually, it would be great if we could turn these fields into something more structured to complement #644. But that’d be a whole usability challenge on par with making the Opening Hours field more usable: openstreetmap/iD#974.