Make it apparent that the site is open-source

[bndnio]

It's not immediately clear that our website is open source, and thus we are probably loosing opportunities to engage with and receive input from local software devs that could volunteer/contribute.

Suggested Approach:
Explore how the Footer element is used in our application, and likely add the GitHub icon into it.
It's sometimes called a "GitHub corner", some example implementations can be found here: https://tholman.com/github-corners/

Acceptance Criteria:

• A Github icon is shown in the bottom right of every page
• Icon is able to be focused on via tab selection
• Element has necessarily properties to make it accessible (has content to read for a screen reader)
• If we're importing an icon to do so, we're not significantly increasing our bundle/application size

[bndnio]

@seanda