[Plugin] April Fools Star Wars joke

[seshrs]

Context

Following up on my stated goal in #197: I think it would be funny if students saw a different joke each year 😅

This commit adds a new way for students to view the spec: As a Star Wars opening crawl!

Validation

Visit: https://preview.sesh.rs/previews/eecs485staff/primer-spec/256/?enable_april_fools_star_wars=1

Button appears next to settings button
Clicking the button opens a popover.
Star Wars crawl

https://github.com/eecs485staff/primer-spec/assets/12139762/47a7de77-c40b-4207-a9ce-7001142e60b3

https://github.com/eecs485staff/primer-spec/assets/12139762/a0821a1d-cd4b-40a1-b6c9-ddeccc87094b

https://github.com/eecs485staff/primer-spec/assets/12139762/2584dfe2-32fe-4df1-9eb7-773dc944685a

[github-actions[bot]]

The spec from this PR is available at https://preview.sesh.rs/previews/eecs485staff/primer-spec/256/.

^{(Available until Tue Feb 06 2024.)}

[seshrs]

Looks like the popover is incorrectly styled in dark mode, this might affect the language selector too…