Dead clicks are created when users navigate back in a MPA

For MPA, dead clicks are created if users click a link and then navigates back. The [bfcache](https://web.dev/articles/bfcache) preserves the timer state and resumes it when the user navigates back and the page is restored from bfcache. We should probably clear our dead click timers when the page is restored from bfcache (and maybe even when it begins to enter bfcache).