Add new region for subsites extras module menu

[markconroy]

Tasks

• Add region in localgov_base.info.yml
• Add region to header.html.twig (probably just inside closing header tag so it can be optionally full width)
• Add toggle button for expanding/collapsing menu on small screens
• Add JS to header.js to support the toggling
• Provide some basic default styles via custom variables for menus in this region

[rupertj]

I've started a branch for this. There's a daft PR here: #542