Makes the default publish date editable (in place of just a placeholder)

closes #3204
- remove original `pubishedAtValue` attribute
- renamed `publishedAtPlaceholder` to `pubishedAtValue`
`publishedAtPlaceholder ` was renamed to `publishedAtValue`, so leaving this would have meant a duplicate property.

And the reason I opted to rename placeholder to value (in place of keeping this) is because if there is no date set the value would be blank. Using the functionally formally known as `publishedAtPlaceholder` defaults to the current date if a `published_at` value is not present.