Make command-panel's editor have -webkit-flex: 1

We moved the setting of flex on editors to the panes stylesheet
previously, but here is another context where we need flex to be
set.

static/command-panel.css

@@ -122,6 +122,7 @@
 
 .command-panel .prompt-and-editor .editor {
   position: relative;
+  -webkit-flex: 1;
 }
 
 .command-panel .prompt-and-editor {