Mainly taken from the privacy policy page
UI in wp-admin/privacy.php.
Some items remain,
including adding the custom post status,
and adding the href value.

New option name is consistent with the naming structure for static
special pages.

Refactored to move the `add_settings_error` code to a separate method.  Hooked that method to `admin_notices` action.  Now settings errors appear both when saving and upon first loading the Reading page.

Modified the "in the trash" error to link a link to the Page's Trash UI.

This new strategy works for both the classic and Gutenberg editors.

Instead of registering multiple callbacks to the `admin_init`, this commit registers `init_admin()`.  Then the new callback method handles the order of calling each of the tasks.

This commit is more performant by O(n-1):

1. Decreases the processing time to do `add_action` as it only have to invoke it once.
2. Decreases the processing time when `do_action` as it only has to invoke one callback.

It's more readable, as we can quickly read that the callback intent is to initialize the admin.

This comment handles excluding special static pages ( i.e. `page_on_front`, `page_for_posts`, and `wp_page_for_privacy_policy`, and `page_for_offline` ) from `wp_dropdown_pages`:

1. When selected, the offline page is excluded from the other dropdowns, preventing it from being selected as anything else but the offline page.
2. The homepage, posts page, and privacy page are all excluded from the Offline Page's dropdown.

This commit also includes slight performance improvements by:

1. Initializing static pages on init.
2. Caching the offline page ID within the object itself, while also providing for a hard request (i.e. when true, run `get_option`).

Using the Privacy code, this commit adds the HTML and create new page functionality.  A unique action is added to the Reading URL.  Upon clicking the "create new page" link, the new functionality inserts a new page and assigns it as the Offline Page.

To be consistent with the other special pages (homepage, posts page, and privacy), this commit re-enables the page attributes.

This commit excludes the offline page from all search and the menu builders including Appearance > Menus and Customizer's Menus.

…downs.

This commit excludes the offline page from rendering on the frontend.  A 404 is served up.

When using plain permalinks, e.g. https://example.com?page_id=28, it will find and render the offline page.

This is an incompletion strategy per conversations in Issue #23 @see #23 (comment).

Grouped the tasks and split into separate classes, as the previous class design was doing too much.

This commit improves the excluder by:

1. Using parse_query.
2. Using $query->is_admin.
3. Checking for the offline page's query first.  If yes, setting a 404 on the frontend.

See #28 (review)

We want to include all special pages, as we use this collection to remove these special pages from the page select dropdowns.

…e page dropdown

and that the offline page is included and selected.

This change was requested by Alberto. He's right as the previous label could be confusing as more than one page can be served in offline.  But this specific page is the default offline status page when the one requested is not available.

Being mindful of performance, this commit checks if the "post__not_in" has a value.  If yes, then it merges with the offline page ID and then filters for only the unique values.

Else, it sets the offline ID only, thereby skipping the `array_merge` and `array_unique`.

The code is more complex.  But it serves to only run the the array functions when needed.