Options: make docs links point to the appropriate version

Until now the help button pointed to the latest version of the
"features" document, which might contain information about features
which haven't been launched yet. This change adapts this link and the
link to the "OP indicator" documentation so they point to the document
corresponding to the version installed (or the latest version if the
extension has been loaded in developer mode).

Change-Id: I2a99886b67d11da00214600620bd26f28ff1bdce
diff --git a/src/common/extUtils.js b/src/common/extUtils.js
index a1b4d48..1777c87 100644
--- a/src/common/extUtils.js
+++ b/src/common/extUtils.js
@@ -11,3 +11,9 @@
   var manifest = chrome.runtime.getManifest();
   return ('version' in manifest) && manifest.version != '0';
 }
+
+// Returns the extension version
+export function getExtVersion() {
+  var manifest = chrome.runtime.getManifest();
+  return manifest?.version;
+}