Configforms list items with a pop-up menu action button weren't styling

correctly in Djblets 2.0. We had changed the structure of pop-up menus,

but didn't change the code  setting up that structure.
This change adds the new structure, and also fixes up some styling

around the menu.

Unit tests passed.
Tested some of our UI that uses this feature. Saw that, instead of a broken

transparent menu, I had a menu in the same appearance as in Djblets 1.0.