Add CC sticky sidebar headers feature

This commit introduces a feature which makes the headers in the CC
sidebar remain visible (by sticking at the top) when scrolling down.

The idea was taken from the suggestion made at pekb/thread/60784834.

Change-Id: I6bbab04da855dbfa35ff8f8b1bbbc6d15946c36c
diff --git a/src/_locales/en/messages.json b/src/_locales/en/messages.json
index 24b93c9..a02a2cb 100644
--- a/src/_locales/en/messages.json
+++ b/src/_locales/en/messages.json
@@ -59,6 +59,10 @@
     "message": "Enables the <code class=\"help\" title=\"This flag activates the functionality which allows a PE to move threads from a forum where they are currently a PE to another forum in the Community Console.\">enableMoveThread</code> Community Console flag.",
     "description": "Feature checkbox in the options page"
+  "options_stickysidebarheaders": {
+    "message": "Make the headers in the Community Console sidebar stick at the top (+info at <code>pekb/thread/60784834</code>).",
+    "description": "Feature checkbox in the options page"
+  },
     "message": "Increase contrast between read and unread threads in the Community Console.",
     "description": "Feature checkbox in the options page"