UI improvements

Collapse the activity contexts by default and show a summary of the logs.
![elmlogger7](https://cloud.githubusercontent.com/assets/2818455/5291098/1974b9ae-7b03-11e4-8688-8718e723fed4.png)

Using CSS to count things seems odd. At the very least we could switch to using data-* attributes so that we have real data instead of using CSS.

But even then, this might be doable more easily (though I'm not sure about perf) by using jQuery and CSS selectors. E.g. maybe you could do something like `logHeader.closest("...").find("*[data-loglevel=critical]")` or something like that. But then you'd need to run that query once for each log level. Or maybe query for all of them quickly first, and then figure out how many of each kind?