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/options.html b/src/options.html
index b11393d..5bdb675 100644
--- a/src/options.html
+++ b/src/options.html
@@ -18,6 +18,7 @@
       <input type="checkbox" id="loaddrafts"> <label for="loaddrafts" data-i18n="loaddrafts"></label> <span style="color: gray;" data-i18n="experimental_label"></span><br>
       <input type="checkbox" id="batchduplicate"> <label for="batchduplicate" data-i18n="batchduplicate"></label> <span style="color: gray;" data-i18n="experimental_label"></span><br>
       <input type="checkbox" id="increasecontrast"> <label for="increasecontrast" data-i18n="increasecontrast"></label><br>
+      <input type="checkbox" id="stickysidebarheaders"> <label for="stickysidebarheaders" data-i18n="stickysidebarheaders"></label><br>
     </p>
     <p style="text-align: center;"><button id="save" data-i18n="save"></button></p>
     <script src="options.js"></script>