We were using the same code for the button in both the public and admin
headers, so we're removing the duplication.
Since the menu and the button must go together, and the contents of the
menu are different for different layouts, we're passing these contents
using a block.
Note the ID of the menu was `responsive-menu` in the public section but
`responsive_menu` in the admin section. Since we usually use underscores
for IDs and dashes for classes, we're keeping the one with the
underscore.
Eventhough some of us sentimentals still like the syntax `to_not` the current trend is to move to the new syntax `not_to`.
In this commit we are updating the references of expectations that used `to_not` to `not_to`.
