tour: fix editor layout rendering bug at slow page load

Forcing the CodeEditor take its full parent height,
solves the issue when the code content was loading
slower, on Chrome. Also the yellow background was
added only on each line, and not on the editor,
resulting in a white box while lessons were loading.

Fixes golang/tour#544

Change-Id: Ic94ba7bf284267f5b2e3ea359342ebca1a29f2b1
Reviewed-on: https://go-review.googlesource.com/122463
Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
diff --git a/static/css/app.css b/static/css/app.css
index 1e0f1d8..e2f2ffd 100755
--- a/static/css/app.css
+++ b/static/css/app.css
@@ -267,7 +267,8 @@
     display: none;
 }
 #file-editor .CodeMirror {
-    height: auto;
+    height: 100%;
+    background: #FFFFD8;
 }
 #file-editor .CodeMirror-lines, #file-editor .CodeMirror-gutters {
     background: #FFFFD8;