Tonya Mork 8ff5899348 Widgets: Preserve classic sidebars when switching to a block theme.
When switching to a block theme, classic sidebars were orphaned and their widgets remapping to the `'wp_inactive_widgets'` sidebar . This changeset preserves the sidebars and their widgets, providing a migration path to a block theme without losing the widgets.

Classic sidebars are now:

* Stored in a new theme mod called `'wp_classic_sidebars'`;
* Restored to the `$wp_registered_sidebars` global variable when the `'widgets_init'` action fires (via a new internal function called `_wp_block_theme_register_classic_sidebars()`);
* And marked as `'inactive'` when interacting with sidebars REST API endpoint.

References:
* [https://github.com/WordPress/gutenberg/pull/45509 Gutenberg PR 45509] which adds an option for importing widgets from sidebars into template parts.

Follow-up to [50995], [6334].

Props mamaduka, audrasjb, hellofromTonya, ironprogrammer, jameskoster, joen, matveb, mukesh27, noisysocks, poena, youknowriad.
Fixes #57531.

git-svn-id: https://develop.svn.wordpress.org/trunk@55200 602fd350-edb4-49c9-b593-d223f7449a82
2023-02-03 00:13:52 +00:00
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