Request: option to position toggle at bottom of screen on mobile

**ISSUE**
On narrow viewports the toggle button takes up a significant amount of width. When positioned at the top of the screen on a mobile view it can overlap site logo or menu toggle. Re-designing the mobile header may not be possible or desirable.

While it is possible to position the toggle button stickily at the bottom of the screen by over-riding css the accessibility menu is then hidden as it always appears below the toggle.

A client has asked me to turn off the accessibility toolbar on mobile because of this. I'll re-code it myself to make it work for them but it would be great if we could have a feature that gives us a better option out of the box.

** POSSIBLE SOLUTION**
- Option to position the toggle button at the bottom of the screen (i.e. position: fixed with a bottom offset rather than top)
- If positioned at the bottom of the screen make menu appear above the button when it scrolls out rather than under.

The issue names no files or tests; begin by locating the toggle and accessibility-menu positioning styles and their mobile viewport rules. Verify the bottom-position option and that the menu opens above the toggle without overlap on narrow screens.

Written by the indexing model from the issue text.