Improve the location of the "base-after-navbar" template hook.
This template hook was in a weird position before. It's, in theory, a

useful hook when you want to place something immediately after the

navbar, flush with the navbar, but instead it was in a container further

down that had padding around it.
This change moves it to be right after the navbar, and adds a new block

that the admin UI needed to place its sidebar.

Tested with the new Message of the Day extension in /r/5000