FEATURE: g,j and g,k to navigate to next and prev topic

After visiting a topic list (by tag / category / top level) we track the list

Once a list is tracked the combo `g` `j` can be used to go to the next topic
in the list and `g` `k` to go to previous topic.

This allows you to quickly work through subsets of topics without having
to navigate back to the top level lists

The shortcut does not work in PM lists yet, or search results, both are
under consideration.

config/locales/client.en.yml

@@ -3185,6 +3185,8 @@ en:
         profile: "%{shortcut} Profile"
         messages: "%{shortcut} Messages"
         drafts: "%{shortcut} Drafts"
+        next: "%{shortcut} Next Topic"
+        previous: "%{shortcut} Previous Topic"
       navigation:
         title: "Navigation"
         jump: "%{shortcut} Go to post #"