Add warning message when using old thread design

In threads which are using nested replies, a warning will be shown if
the user has enabled the old thread design to notify them that they
won't work properly.

Fixed: twpowertools:139
Change-Id: I393ac187fef22a4c5f062bf99d84c373011d514b
diff --git a/src/common/extUtils.js b/src/common/extUtils.js
index cabf8cb..39cf196 100644
--- a/src/common/extUtils.js
+++ b/src/common/extUtils.js
@@ -13,3 +13,19 @@
   var manifest = chrome.runtime.getManifest();
   return manifest?.version;
 }
+
+// Get a URL to a document which is part of the extension documentation (using
+// |ref| as the Git ref).
+export function getDocURLWithRef(doc, ref) {
+  return 'https://gerrit.avm99963.com/plugins/gitiles/infinitegforums/+/' +
+      ref + '/docs/' + doc;
+}
+
+// Get a URL to a document which is part of the extension documentation
+// (autodetect the appropriate Git ref)
+export function getDocURL(doc) {
+  if (!isProdVersion()) return getDocURLWithRef(doc, 'HEAD');
+
+  var version = getExtVersion();
+  return getDocURLWithRef(doc, 'refs/tags/v' + version);
+}