implement go-to-last-anchor

src/org/thoughtcrime/securesms/LocalHelpActivity.java

@@ -105,10 +105,18 @@ public class LocalHelpActivity extends PassphraseRequiredActionBarActivity
                 openOnlineUrl("https://github.com/deltachat/deltachat-android/issues");
                 return true;
         }
-
         return false;
     }
 
+    @Override
+    public void onBackPressed() {
+        if (webView.canGoBack()) {
+            webView.goBack();
+        } else {
+            super.onBackPressed();
+        }
+    }
+
     private void openOnlineUrl(String url) {
         try {
             startActivity(new Intent(Intent.ACTION_VIEW, Uri.parse(url)));