Use monospaced font for CodeMirror editor.

Cursor position gets off with non-monospaced font.
STUD-1456.

Vertical alignment of monospace font in advanced settings via position css adjustment

cms/static/sass/views/_settings.scss

@@ -828,7 +828,7 @@
         border: 1px solid $mediumGrey;
         border-radius: 2px;
         background-color: $lightGrey;
-        font-family: 'Open Sans', sans-serif;
+        font-family: $f-monospace;
         color: $baseFontColor;
         outline: 0;
         height: auto;
@@ -840,6 +840,10 @@
           outline: 0;
         }
 
+        .CodeMirror-sizer {
+          top: 4px; /* Vertical alignment for monospace font */
+        }
+
         // editor color changes just for JSON
         .CodeMirror-lines {