Draft: New hamburger menu (mobile-friendly)

[mtmacdonald]

Adding a new mobile-friendly hamburger menu. This is a CSS-only implementation loosely based on this.

Todo:

• [ ] fix topbar overlapping hamburger button on narrow screens
• [x] test / fix on mobile screens (size too small, not stretching to bottom of screen)
• [x] sponsorship banner appears above menu (removed opacity on banner)
• [ ] test scrollbar for long lists
• [ ] hide hamburger from PDF generator
• [ ] close button not clickable on iOS
• [x] put back the content overlay opacity
• [ ] remove ts-nocheck
• [x] adjust styles of menu to fit brand/app
• [x] add new hamburger-style menu
• [x] remove old web table of contents component

[mtmacdonald]

Unfortunately I had to abandon this approach - it worked well on Desktop but the quality on mobile just wasn't good enough. The "clever" CSS had too many side-effects (like not working if the content body had opacity applied).

Replacing it with PR95