Start Date and End Date fields should have structured input fields

[1ec5]

iD’s Start Date and End Date fields are currently typed as date fields. Due to a fortunate bug, they wind up as ordinary text fields that accept the full range of years and allow months and days to be omitted: #563. But this still requires the user to be familiar with ISO 8601-esque syntax; otherwise, they’ll get yelled at by the validator, which isn’t a great experience. Worse, BCE dates aren’t super intuitive in ISO 8601; I’ve spotted start_dates that are off by one due to manual entry.

We should replace the single text field with a series of dropdowns and text fields, similar to what’s in the time slider for setting the date range:

While we’re at it, we should use the standard internationalization APIs to determine which order to show the subfields in and present a separate dropdown for the era, so the user doesn’t have to guess (probably wrongly) how negative years work: #626 (comment).

For users who need to copy-paste already ISO-formatted dates from other sources, there’s always the raw tag editor, which would remain the same.

[1ec5]

The roadheight field type similarly stuffs multiple HTML input fields and dropdowns into a single field. It would be a good model to follow for this enhancement.