FIX: Restore stream position in Safari (PR #9993)

Safari uses an aggressive back/forward cache, which means the app loads very quickly when hitting the Back button. But, in topics with > 30 posts, hitting Back runs post stream calculations too early, which means that users get taken back to an earlier point in the stream, consistently.

Using onpageshow, we can restore the correct location before the post stream calculations take place.

In theory the fix should apply to other browser with aggressive back/forward caching, but in my tests I did not see this in Chrome or Firefox (will test other browsers shortly).