Make fallback date format editable rather than hard coding to US notation

Problem/Motivation

Drupal has a "Fallback date format" which always uses the US notation: MM/DD/YYYY
With Drupal being used a lot in non US projects this is actually a problem since the fallback format can't be edited and is used when a format is not provided.

Example of a module not passing a date format in a date value: https://git.drupalcode.org/project/webform/-/blob/8.x-5.x/src/WebformSub...
In the above line:
($entity->{$name}->value ? $date_formatter->format($entity->{$name}->value) : '')
should be (for example)
($entity->{$name}->value ? $date_formatter->format($entity->{$name}->value ,'long') : '')

Steps to reproduce

1. Install Drupal
2. Visit: /admin/config/regional/date-time
3. See that Fallback date format can't be edited and is using US notation.

Proposed resolution

Updating steps based on suggestions from @alexpott on comment #22 ✨ Make fallback date format editable rather than hard coding to US notation Needs work

• Unlock the fallback date format
• Add special case the fallback ID and unlock it.
• Update \Drupal\system\DateFormatAccessControlHandler::checkAccess() to cope with the new rules.
• We'll also need to update \Drupal\Core\Datetime\Entity\DateFormat::sort() to treat the fallback as locked.
• Add a constant on the DateFormat class that has the id string for the fallback.

Option 1 should be enough for everybody.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet