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/console_inject.js b/src/console_inject.js
index b2ca80e..c97483b 100644
--- a/src/console_inject.js
+++ b/src/console_inject.js
@@ -88,4 +88,8 @@
   if (options.increasecontrast) {
     injectStyles(".thread-summary.read{background: #ecedee!important;}");
   }
+
+  if (options.stickysidebarheaders) {
+    injectStyles("material-drawer .main-header{background: #fff; position: sticky; top: 0; z-index: 1;}");
+  }
 });