bug: avoid page crash

[khiga8]

This PR makes a couple of updates:

• fix bug where page crashes when I go to Notifications from home page. This is a recent issue I've been noticing. I think there might be an infinite mutation loop happening when I go from GitHub home to Notifications. I am unable to hit a breakpoint when I try to debug, but adding observer.disconnect(); call within the callback fixes the issue.
• adds an interval to debounce making the appendAccessibilityInfo updates
• updates the selector we listen to from mutation.target.matches('.markdown-body') to mutation.target.closest('.markdown-body')