Only apply live updates during main query and fix dashboard live updates bug

Resolves #675 

@shadyvb please review
@lukecarbis Notice the usage of this var in [`live-updates.js`](https://github.com/wp-stream/stream/pull/676/files#diff-03ad9ca78da0bb3744c7cd2942a34393R12) though. It's just going to prevent live updates when there is more than 1 param and it only runs on the `toplevel_page_wp_stream` screen.

Maybe it's not 100% bulletproof, but I can't think of any legitimate reason why any plugin would want to add another `$_GET` param to default views of the Stream Records screen. Doing that would be wrong on their part, so I'm not keen to support it :smile: