Add replay button to anniversary video.

Buttons stolen from https://www.mozilla.org/en-US/firefox/independent/
Nope, it gets embedded directly in about:home with the rest of the snippets. The `{{ snippet_id }}` avoids snippets from affecting eachother (which has been a HUGE problem in the past).

@koddsson actually has PR up with an alternative fix that only injects a single snippet into the page, we're just too backed up on other work to review/test/deploy it right now. :(