Introduce a flag to indicate a "fresh site"

[helen]

Closely tied to starter content in #38114, but also potentially generally useful.

Thinking a flag (option) that is set on install and then deleted upon the first run of one of a number of actions. Those actions should be: adding/updating a post or page, changing widgets, or saving customizer settings.

It might make more sense to have different options for each action to target the user better. They could open the customizer and hit save, but still be a newbie in regard to saving posts and widgets.

38533.1.diff​ sets the option to 0 so the option is autoloaded to prevent further db queries.

Not sure about the get_option() in default-filters though, looks like it's first one that runs so early. Currently it's wp_plugin_directory_constants() with get_option('siteurl') so I guess it's fine.

Please review and make additional revs via pushing to feature branch on GitHub. See PR: ​https://github.com/xwp/wordpress-develop/pull/188

In 38991:

Customize: Introduce starter content and site freshness state.

A theme can opt-in for tailored starter content to apply to the customizer when previewing the theme on a fresh install, when fresh_site is at its initial 1 value. Starter content is staged in the customizer and does not go live unless the changes are published. Initial starter content is added to Twenty Seventeen.

• The fresh_site flag is cleared when a published post or page is saved, when widgets are modified, or when the customizer state is saved.
• Starter content is registered via starter-content theme support, where the argument is an array containing widgets, posts, nav_menus, options, and theme_mods. Posts/pages in starter content are created with the auto-draft status, re-using the page/post stubs feature added to nav menus and the static front page controls.
• A get_theme_starter_content filter allows for plugins to extend a theme's starter content.
• Starter content in themes can/should re-use existing starter content items in core by using named placeholders.
• Import theme starter content into customized state when fresh site.
• Prevent original_title differences from causing refreshes if title is present.
• Ensure nav menu item url is set according to object when previewing.
• Make sure initial saved state is false if there are dirty settings without an existing changeset.
• Ensure dirty settings are cleaned upon changeset publishing.

Props helen, westonruter, ocean90.
Fixes #38114, #38533.

Looks like this is getting populated as 1 on existing sites, I guess we should populate as 0 in that case.

OK, so populate_options() will populate the options defined when upgrading in addition to when installing. Maybe the fresh_site flag should be set in wp_install() instead. This would ensure it only gets added for new installs and only once.

See 38533.2.diff​.

In 39076:

Only set fresh_site during an actual fresh install.

props westonruter.
fixes #38533.

#38552 was marked as a duplicate.

In 39276:

Customize: Add unit tests for importing theme starter content.

Props welcher, westonruter.
See #38114, #38533, #38615.
Fixes #38540.