Add support for the default_page_template_title filter in page-attributes meta-box.

[kallehauge]

Description

I have wrapped the availableTemplates call to wp_get_theme()->get_page_templates(...) in a PHP function to directly add the default template into the array so we can use the already existing default_page_template_title filter instead of hardcoding it in the editor/components/page-attributes/template.js file.

More about the reasoning can be found in #6946

Fixes #6946

How has this been tested?

The change is very isolated and will only have any effect on the page-attributes meta box. I have run manual testing by testing the following scenarios:

• Make sure we only append the default template logic if the page have any templates (done by using the Twentysixteen theme).
• Make a child theme of Twentysixteen and add a template.
• Save / update the post to make sure the value is "saved correctly" (more precisely the lack of a value since the value for the default template is an empty string).

The environment was a clean vanilla install of WordPress 4.9.6 with Twentysixteen 1.5 run on a custom Docker installation (PHP 7.1 / Apache 2.4) with a manually run npm install and npm run dev.

Types of changes

Added new functionality to support already existing functionality in WordPress.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[danielbachhuber]

Thanks @kallehauge ! I've pushed a small improvement in b59cd28

[kallehauge]

Awesome @danielbachhuber ! And nice improvements to the code 💪

[kallehauge]

Oi! @danielbachhuber I think I just realized a scenario your more streamlined code do not take into account (the comment here).

The '' => Default template array entry should not be added if the array is empty (there are no additional templates to choose from).
The reason for this is that the template <select> will not show up if the array is empty and it will just be assumed that the default template will be used. But if we always add Default template to the array, then the <select> will always appear.

[danielbachhuber]

The '' => Default template array entry should not be added if the array is empty (there are no additional templates to choose from).

Ah, makes sense. Want to submit a new PR with that change?

[kallehauge]

Sure @danielbachhuber 👍