Update subscribe button

[bakkot]

Probably best to review the second commit individually; the first one is a git revert of aaa04a4 so that the second one makes more sense.

The idea is that whenever the channelContainer element is updated (per a MutationObserver) we check to see if the handle has changed, and if so replace the subscribe/download buttons with new buttons.

[farcaller]

Is this actually functional? Since this PR the dlButton.setAttribute('data-id', videoId) is gone, so no data-id is set on the button, but the sendDownload() still references it.

I've tried using the extension off the master branch and the button doesn't work for me.

Edit: While that seems broken it's only triggered through the code path that's only accessible when you're not logged in