Add support for quick keys

[simonminer]

Hi @suellybelly,

Here's an eleventh hour PR to add support for the quick keys @dorlync set out. Specifically:

• Press h/H to move forward/backward through headings.
• Press k/K to move forward/backward through links.
• Press r/R to move forward/backward through page regions and landmarks.
• Press f/F to move forward/backward through form controls.
• Press b/B to move forward/backward through buttons.
• Press l/L to move forward/backward through lists.

Elements need to be focusable for the quick keys to work on them. Right now, the application only contains a subset of the elements supported by quick keys (headings, links, and a single list), but these appear to work properly.

Specific changes to support quick keys include the following.

• Adding the following files to the repo
  • build/assets/js/modules/quick-key.js - JS class to represent a single quick key
  • build/assets/js/modules/quick-key-manager.js - JS clas to manage a set of quick keys
  • build/assets/js/quick-key-setup.js - JS module to load quick keys listed above.
• Add a <script> tag to build/index.html to load the quick keys into the application.
• Add a sentence explaining how to use applicable quick keys to the quiz overlay.

FYI - the files listed above were copied from a separate repo I've been working on to generalize the Inclusiville simulated screen reader.

Hope this feature can be added before the project is submitted. Thanks.

• @simonminer