Add accessibility toggle button to navigation bar

[oliverfoster]

• To allow touch devices to toggle accessibility fully rather than having it always on.
• Must be the first tabbable item for desktops
• Need to add an appropriate icon to the vanilla fonts

[chris-steele]

I've spent a while reviewing this and conclude that this is not practical at the moment. One issue is that the order of setup in accessibility.js comes prior to the creation of the navigation bar. A further issue is the incompatibility with adapt-contrib-languagePicker.

After discussion we will tackle this as two separate issues, amending just the toggle button in this issue and adding as a dependency a second issue to resolve the required changes to the language picker.