Themes: Add support for Update URI header.

This allows third-party themes to avoid accidentally being overwritten with an update of a theme of a similar name from the WordPress.org Theme Directory.

Additionally, introduce the `update_themes_{$hostname}` filter, which third-party themes can use to offer updates for a given hostname.

If set, the `Update URI` header field should be a URI and have a unique hostname.

Some examples include:

* `https://wordpress.org/themes/example-theme/`
* `https://example.com/my-theme/`
* `my-custom-theme-name`

`Update URI: false` also works, and unless there is code handling the `false` hostname, the theme will never get an update notification.

If the header is present, the WordPress.org API will currently only return updates for the theme if it matches the following format:

* `https://wordpress.org/themes/{$slug}/`
* `w.org/theme/{$slug}`

If the header has any other value, the API will not return a result and will ignore the theme for update purposes.

Follow-up to [50921].

Props dd32, meloniq, costdev, audrasjb, DavidAnderson, markjaquith, DrewAPicture, mweichert, design_dolphin, filosofo, sean212, nhuja, JeroenReumkens, infolu, dingdang, joyously, earnjam, williampatton, grapplerulrich, markparnell, apedog, afragen, miqrogroove, rmccue, crazycoders, jdgrimes, damonganto, joostdevalk, jorbin, georgestephanis, khromov, GeekStreetWP, jb510, Rarst, juliobox, Ipstenu, mikejolley, Otto42, gMagicScott, TJNowell, GaryJ, knutsp, mordauk, nvartolomei, aspexi, chriscct7, benoitchantre, ryno267, lev0, gregorlove, dougwollison, leemon, SergeyBiryukov.
See #14179, #23318, #32101.
Built from https://develop.svn.wordpress.org/trunk@53933


git-svn-id: http://core.svn.wordpress.org/trunk@53492 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-theme.php

@@ -23,6 +23,7 @@ final class WP_Theme implements ArrayAccess {
 	 *
 	 * @since 3.4.0
 	 * @since 5.4.0 Added `Requires at least` and `Requires PHP` headers.
+	 * @since 6.1.0 Added `Update URI` header.
 	 * @var string[]
 	 */
 	private static $file_headers = array(
@@ -39,6 +40,7 @@ final class WP_Theme implements ArrayAccess {
 		'DomainPath'  => 'Domain Path',
 		'RequiresWP'  => 'Requires at least',
 		'RequiresPHP' => 'Requires PHP',
+		'UpdateURI'   => 'Update URI',
 	);
 
 	/**
@@ -844,9 +846,11 @@ final class WP_Theme implements ArrayAccess {
 	 *
 	 * @since 3.4.0
 	 * @since 5.4.0 Added support for `Requires at least` and `Requires PHP` headers.
+	 * @since 6.1.0 Added support for `Update URI` header.
 	 *
 	 * @param string $header Theme header. Accepts 'Name', 'Description', 'Author', 'Version',
-	 *                       'ThemeURI', 'AuthorURI', 'Status', 'Tags', 'RequiresWP', 'RequiresPHP'.
+	 *                       'ThemeURI', 'AuthorURI', 'Status', 'Tags', 'RequiresWP', 'RequiresPHP',
+	 *                       'UpdateURI'.
 	 * @param string $value  Value to sanitize.
 	 * @return string|array An array for Tags header, string otherwise.
 	 */
@@ -896,6 +900,7 @@ final class WP_Theme implements ArrayAccess {
 			case 'Version':
 			case 'RequiresWP':
 			case 'RequiresPHP':
+			case 'UpdateURI':
 				$value = strip_tags( $value );
 				break;
 		}