FIX: switch panel back to forum last known url (#31092)

Previously the return to forum link would automatically take you to the
forum homepage, however this was not intended functionality. We should
attempt to take the user to their last viewed forum page.

This change fixes a bug in how we determined what the destination link
should be.

Internal ref: /t/110508

plugins/chat/spec/system/separate_sidebar_mode_spec.rb

@@ -221,7 +221,7 @@ RSpec.describe "Separate sidebar mode", type: :system do
         expect(channel_browse_page.component).to be_present
 
         sidebar_component.switch_to_main
-        expect(page).to have_current_path("/discuss/")
+        expect(page).to have_current_path("/discuss/about")
       end
     end
   end