Customizer: Avoid hiding 'Remove' buttons unrelated to custom headers.

props rzen, ehg.
fixes #27848.

Built from https://develop.svn.wordpress.org/trunk@28266


git-svn-id: http://core.svn.wordpress.org/trunk@28094 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Dead code in `wp-admin/includes/file.php`:
* In `wp_handle_upload()` and `wp_handle_sideload()`, `$ext` gets conditionally reset... and then is never used.
* In `request_filesystem_credentials()`, `$password` is initialized as an empty string. The variable is never used.

See #27882.


Built from https://develop.svn.wordpress.org/trunk@28268


git-svn-id: http://core.svn.wordpress.org/trunk@28096 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Customizer: Only tie header button action events to the header controls.

props danielbachhuber.
fixes #28046. see #27848.

Built from https://develop.svn.wordpress.org/trunk@28269


git-svn-id: http://core.svn.wordpress.org/trunk@28097 1a063a9b-81f0-0310-95a4-ce76da25c4cd