Closes#4379
- Moves the bottom notification outlet _outside_ the `<main>` element which allows notifications to show above the nav bar
- Removes (the now unnecessary) code which counter-positioned the bottom notifications when a Settings Menu was opened.
The way these notifications look or are positioned at various viewport sizes **has not changed**.
Continues on from #4401
- Reformats the Sass to match our new style (property order, comments, & white space)
- Changes the modal container to a class from an ID (and adjust tests)
No issue
I'm not sure why, but @extend's here using placeholder selectors, not classes, work.
This fixes the notification and modal styles, which meant changing how the default button selector works. It is now a placeholder, and a real class extends that. The modal styles extended the default button style, which can now happen.
