AK: Stop allowing implicit downcast with RefPtr and NonnullRefPtr

We were allowing this dangerous kind of thing:

RefPtr<Base> base;
RefPtr<Derived> derived = base;

This patch changes the {Nonnull,}RefPtr constructors so this is no
longer possible.

To downcast one of these pointers, there is now static_ptr_cast<T>:

RefPtr<Derived> derived = static_ptr_cast<Derived>(base);

Fixing this exposed a ton of cowboy-downcasts in various places,
which we're now forced to fix. :^)

Applications/ChanViewer/ThreadCatalogModel.h

@@ -63,5 +63,5 @@ private:
 
     String m_board { "g" };
     JsonArray m_catalog;
-    RefPtr<Core::HttpJob> m_pending_job;
+    RefPtr<Core::NetworkJob> m_pending_job;
 };