Order comment threads in pull request Activity section based on most recent reply

Some pull requests can have a ton of comment threads on them.  Often, a user wants to be able to easily see any new replies to these comment threads.

Three ways of going about that are:

1. Getting notified about the replies through email.  Not ideal because:
   1. Users often batch emails to avoid getting too many from Bitbucket, but they still want to be able to quickly see new comment thread replies when they are actively reviewing a pull request.
2. Looking at the diff for the file that holds the comment thread.  Not ideal because:
   1. Further commits may have hidden the comment thread, even though the comments may not have been fully addressed.
   2. You have to remember the files with comment threads you want to check up on, as well as the location of the comment threads on those files.
3. Scroll through your entire Activity and look for new replies that you haven't seen before.

The third option can be unmanageable on pull requests with many comment threads, as you may have to scroll for an unreasonably long time to make it through the entire Activity (and the page loads up more Activity as you scroll, which also takes time).  

However, a simple improvement would be to reorder the Activity as new replies are posted on comment threads.  This would mean that if you have a comment thread that was buried at the very beginning of the Activity, if someone added a new reply to it, it would jump to the top of the Activity.

With such changes in place, you could scroll through your Activity until you get to a comment that has no new replies.  Everything above that comment has new replies.