Add pa11y scanner

[beechnut]

Previously, we did accessibility scanning for every file in every push / build / PR, which took > 25 mins each run.

Now, we more carefully curate the files that need to be scanned, to limit the scan time.

This PR:

• Adds a per-page pa11y scan, which lists pages that have changed or whose layouts have changed.
• Adds a site-wide pa11y scan: if files that affect the whole site change, take a sample of files from across the site to check
• Incidentally deletes an unused layout
• Pins the glob package to 8, because 9 and higher breaks something with the SVG Sprite package.

Commentary:

This commit gets us to the "Make it work" stage, not yet addressing "Make it right" or "Make it fast".

The code also reads a little weird, and I'm not sure a conditional "eleventy.after" event is the right design for this. I think one more refactor here would be wise.

Closes #3875

[caleywoods]

I added some non blocking suggestions related to readability, this feels like a great start Matt!