Add EDTF start and end date fields

[1ec5]

Added Start Date (EDTF) and End Date (EDTF) fields that automatically appear beneath the Start Date and End Date fields when the corresponding non-EDTF field is filled in.

Outdated screenshot

This is just a plain field. Future enhancements can be tracked separately. For example, we could consolidate the EDTF fields with the Start Date and End Date fields, having them appear only when clicking the ➕ button, similar to how the localized name fields work. We can also implement validation and human-readable suggestions as a follow-up. ✔️

Fixes OpenHistoricalMap/issues#645. Depends on #185.

[1ec5]

I removed the separate fields for the start_date:edtf and end_date:edtf keys in favor of extending the date field type to manage *:edtf subkeys in the same manner that a localized field type manages language subkeys. The EDTF field only appears if the subkey is already set or if the user manually presses the ➕ button. This keeps the EDTF field from getting in the way, since two of them are right at the top of the inspector.

[1ec5]

I added a validation rule for malformed EDTF dates:

[1ec5]

I added another validation rule that checks whether start_date falls within the range specified by start_date:edtf and whether end_date falls within the range specified by end_date:edtf. If not, it suggests the start and end of the EDTF date’s range as substitutes in the main date tag: