Adjust rubocop config to allow `[]` delimiters for %i/%w/%W

These three things return arrays, so we like the syntax to be
array-like.
Add missing semicolon and comma in keyboard-shortcuts.js

As pointed out by jslint.
Use aria-selected attribute for keyboard shortcuts

Instead of using an arbitrary CSS class for showing which element is
focused, we can use the aria-selected attribute which will benefit
people using assistive technologies.
Set focus on elements when using keyboard shortcuts

This will help assistive technologies follow the action.
Improve alt text of snapshot thumbnails

The default alt text for these images was the filename, which is just a
bunch of letters and numbers. This wasn't helping anyone. Using the
title and viewport width is much better and will benefit people using
assistive technologies.
Wrap breadcrumb trail in <nav>

This will benefit people using assistive technologies.
Improve edit viewport link title

Using the actual viewport size in the link title improves the
accessibility of the page and benefits people using assistive
technologies.