PRO-6591: parked fields overrides

[myovchev]

Summary

Ensure slug is not modified for parked pages when not configured in _defaults. Migrate any site having "home" parked doc with slug different than /,

What are the specific steps to test this change?

Slug of parked page can't be changed if not present in the _defaults configuration.

What kind of change does this PR introduce?

(Check at least one)

• Bug fix
• New feature
• Refactor
• Documentation
• Build-related changes
• Other

Make sure the PR fulfills these requirements:

• It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• The changelog is updated
• Related documentation has been updated
• Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[linear]

PRO-6591 The automatic translation extension translates the slugs of parked pages like the home page and the import export translation module may also do it, this breaks sites