Add option to choose the thread page design in the CC

This feature allows users to choose whether to show the old thread page
design or the new one.

This CL also adds an experimental feature to activate the nested replies
experiment in the Community Console (only available in the Canary
channel). It has been added as an experimental feature because the
nested replies feature doesn't work yet in the Community Console.

Change-Id: Ifc371848b55f450bedf22d8e35fb4348fffa6c63
diff --git a/src/static/_locales/en/messages.json b/src/static/_locales/en/messages.json
index f6ac4be..23e03f6 100644
--- a/src/static/_locales/en/messages.json
+++ b/src/static/_locales/en/messages.json
@@ -159,6 +159,22 @@
     "message": "Show per-forum activity in profiles.",
     "description": "Feature checkbox in the options page"
   },
+  "options_interopthreadpage": {
+    "message": "Show <span id='interopthreadpage_mode--container'></span> in the Community Console.",
+    "description": "Feature checkbox in the options page. \"<span id='interopthreadpage_mode--container'></span>\" will be substituted with a selector which allows users to select whether they want the new thread page design or the old one."
+  },
+  "options_interopthreadpage_mode_previous": {
+    "message": "the old thread page design",
+    "description": "Select option added in #interopthreadpage_mode--container, in the options_interopthreadpage string"
+  },
+  "options_interopthreadpage_mode_next": {
+    "message": "the new thread page design",
+    "description": "Select option added in #interopthreadpage_mode--container, in the options_interopthreadpage string"
+  },
+  "options_nestedreplies": {
+    "message": "Turn on the nested replies feature in the Community Console.",
+    "description": "Feature checkbox in the options page"
+  },
   "options_save": {
     "message": "Save",
     "description": "Button in the options page to save the settings"