File Editors: Introduce sandboxed live editing of PHP files with rollbacks for both themes and plugins.

* Edits to active plugins which cause PHP fatal errors will no longer auto-deactivate the plugin. Supersedes #39766.
* Introduce sandboxed PHP file edits for active themes, preventing accidental whitescreening of a user's site when introducing a fatal error.
* After writing a change to a PHP file for an active theme or plugin, perform loopback requests on the file editor admin screens and the homepage to check for fatal errors. If a fatal error is encountered, roll back the edited file and display the error to the user to fix and try again.
* Introduce a secure way to scrape PHP fatal errors from a site via `wp_start_scraping_edited_file_errors()` and `wp_finalize_scraping_edited_file_errors()`.
* Moves file modifications from `theme-editor.php` and `plugin-editor.php` to common `wp_edit_theme_plugin_file()` function.
* Refactor themes and plugin editors to submit file changes via Ajax instead of doing full page refreshes when JS is available.
* Use `get` method for theme/plugin dropdowns.
* Improve styling of plugin editors, including width of plugin/theme dropdowns.
* Improve notices API for theme/plugin editor JS component.
* Strip common base directory from plugin file list. See #24048.
* Factor out functions to list editable file types in `wp_get_theme_file_editable_extensions()` and `wp_get_plugin_file_editable_extensions()`.
* Scroll to line in editor that has linting error when attempting to save. See #41886.
* Add checkbox to dismiss lint errors to proceed with saving. See #41887.
* Only style the Update File button as disabled instead of actually disabling it for accessibility reasons.
* Ensure that value from CodeMirror is used instead of `textarea` when CodeMirror is present.
* Add "Are you sure?" check when leaving editor when there are unsaved changes.

Supersedes [41560].
See #39766, #24048, #41886.
Props westonruter, Clorith, melchoyce, johnbillion, jjj, jdgrimes, azaozz.
Fixes #21622, #41887.


git-svn-id: https://develop.svn.wordpress.org/trunk@41721 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/admin-ajax.php

@@ -64,7 +64,7 @@ $core_actions_post = array(
 	'parse-media-shortcode', 'destroy-sessions', 'install-plugin', 'update-plugin', 'crop-image',
 	'generate-password', 'save-wporg-username', 'delete-plugin', 'search-plugins',
 	'search-install-plugins', 'activate-plugin', 'update-theme', 'delete-theme', 'install-theme',
-	'get-post-thumbnail-html', 'get-community-events',
+	'get-post-thumbnail-html', 'get-community-events', 'edit-theme-plugin-file',
 );
 
 // Deprecated