Add process_visible() call to maybe_select_closest().

This fixes a regression where we removed a call to
unread_ops.process_visible() inside of stream_list.js.  Now
we call it from within narrow.activate() in the the
maybe_select_closest() callback.

static/js/narrow.js

@@ -196,6 +196,7 @@ exports.activate = function (raw_operators, opts) {
                 // narrowing
                 message_viewport.set_message_offset(then_select_offset);
             }
+            unread_ops.process_visible();
         }
     }