set/remove @disabled on navigation buttons

make speaker notes keyboard accessible

Add selection colour for Mozilla Firefox

Found that by running css-lint on the code

Places div.speaker-notes in default kbd Tab order, and when focused,
prevent slide navigation with up/down arrows, allowing up/down arrow
keys to scroll div.speaker-notes.

Keeps disabled buttons out of kbd tab order and indicates to assistive
tech like screen readers that button is disabled. Otherwise buttons
that aren’t enabled remain in kbd Tab order and screen readers announce
them as buttons, but they don’t work.

Simplify full setup

More natural zooming on block level elements

Defines the highlight colour when selecting slide content using Mozilla
Firefox to match other browsers, instead of being the default colour.

Switching a `display: block` element to `display: inline-block` allows calculating the bounds based on the contents of the div rather than the entire container (which is often `width: 100%`).

This provides a much more natural zoom, especially for paragraphs and code examples.

`./*.md` didn't work `*.md` does work

Fixes #1549, now UA match is limiting out MS Edge and Chrome detection is improved. Also improved UserAgent access by storing it in to a local variable, it was accessed so many times in different places.