Add kill switch mechanism

This code implements the kill switch mechanism in the extension. This is
explained in //src/killSwitch/ and in the design doc:

Bug: twpowertools:64

Change-Id: Ia993c78035bba7038aafd53d156f20954217e86f
diff --git a/src/static/_locales/en/messages.json b/src/static/_locales/en/messages.json
index f435664..10781fa 100644
--- a/src/static/_locales/en/messages.json
+++ b/src/static/_locales/en/messages.json
@@ -15,6 +15,10 @@
     "message": "Get infinite scroll and other enhancements in the Google Forums and the Community Console",
     "description": "The extension's description"
+  "options_killswitchwarning": {
+    "message": "One or more options have been force disabled because they might not be working properly. Please go to the <a href='' rel='noreferrer noopener' action='_blank'>twpowertools-discuss Google Group</a> to learn more about why some options have been force disabled.",
+    "description": "Warning shown in the options page if a feature has been remotely force disabled via the kill switch mechanism. This happens when a feature doesn't work properly and might be doing dangerous things, and the maintainer or an authorized user remotely force disables the feature."
+  },
   "options_list": {
     "message": "Enable infinite scrolling in thread lists.",
     "description": "Feature checkbox in the options page"