Editor: Add CodeMirror-powered code editor with syntax highlighting, linting, and auto-completion.

* Code editor is integrated into the Theme/Plugin Editor, Additional CSS in Customizer, and Custom HTML widget. Code editor is not yet integrated into the post editor, and it may not be until accessibility concerns are addressed.
* The CodeMirror component in the Custom HTML widget is integrated in a similar way to TinyMCE being integrated into the Text widget, adopting the same approach for integrating dynamic JavaScript-initialized fields.
* Linting is performed for JS, CSS, HTML, and JSON via JSHint, CSSLint, HTMLHint, and JSONLint respectively. Linting is not yet supported for PHP.
* When user lacks `unfiltered_html` the capability, the Custom HTML widget will report any Kses-invalid elements and attributes as errors via a custom Kses rule for HTMLHint.
* When linting errors are detected, the user will be prevented from saving the code until the errors are fixed, reducing instances of broken websites.
* The placeholder value is removed from Custom CSS in favor of a fleshed-out section description which now auto-expands when the CSS field is empty. See #39892.
* The CodeMirror library is included as `wp.CodeMirror` to prevent conflicts with any existing `CodeMirror` global.
* An `wp.codeEditor.initialize()` API in JS is provided to convert a `textarea` into CodeMirror, with a `wp_enqueue_code_editor()` function in PHP to manage enqueueing the assets and settings needed to edit a given type of code.
* A user preference is added to manage whether or not "syntax highlighting" is enabled. The feature is opt-out, being enabled by default.
* Allowed file extensions in the theme and plugin editors have been updated to include formats which CodeMirror has modes for: `conf`, `css`, `diff`, `patch`, `html`, `htm`, `http`, `js`, `json`, `jsx`, `less`, `md`, `php`, `phtml`, `php3`, `php4`, `php5`, `php7`, `phps`, `scss`, `sass`, `sh`, `bash`, `sql`, `svg`, `xml`, `yml`, `yaml`, `txt`.

Props westonruter, georgestephanis, obenland, melchoyce, pixolin, mizejewski, michelleweber, afercia, grahamarmfield, samikeijonen, rianrietveld, iseulde.
See #38707.
Fixes #12423, #39892.

Built from https://develop.svn.wordpress.org/trunk@41376


git-svn-id: http://core.svn.wordpress.org/trunk@41209 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/common-rtl.css

@@ -2200,7 +2200,7 @@ h1.nav-tab-wrapper, /* Back-compat for pre-4.4 */
 	visibility: visible;
 }
 
-#template div {
+#template > div {
 	margin-left: 190px;
 }
 
@@ -3015,13 +3015,18 @@ img {
 #template textarea {
 	font-family: Consolas, Monaco, monospace;
 	font-size: 13px;
-	width: 97%;
 	background: #f9f9f9;
 	-moz-tab-size: 4;
 	-o-tab-size: 4;
 	tab-size: 4;
 }
 
+#template textarea,
+#template .CodeMirror {
+	width: 97%;
+	height: calc( 100vh - 220px );
+}
+
 /* rtl:ignore */
 #template textarea,
 #docs-list {
@@ -3032,6 +3037,25 @@ img {
 	width: 97%;
 }
 
+#file-editor-linting-error {
+	margin-top: 1em;
+	margin-bottom: 1em;
+}
+#file-editor-linting-error > .notice {
+	margin: 0;
+	display: inline-block;
+}
+#file-editor-linting-error > .notice > p {
+	width: auto;
+}
+#template .submit {
+	margin-top: 1em;
+	padding: 0;
+}
+
+#template .submit input[type=submit][disabled] {
+	cursor: not-allowed;
+}
 #templateside {
 	float: left;
 	width: 190px;
@@ -3585,12 +3609,13 @@ img {
 		margin-top: -5px;
 	}
 
-	#template div {
+	#template > div {
 		float: none;
 		margin: 0;
 		width: auto;
 	}
 
+	#template .CodeMirror,
 	#template textarea {
 		width: 100%;
 	}