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)

You know, I was thinking. Why computer this every time it's expanded/collapsed? Presumably the server already knows all this information and can dump it out into some `data-*` attributes ahead of time, no? Or maybe even have the server pre-render the "collapsed" rows but with CSS to hide it. In other words it would render:

    <table>
        <tr>... stuff goes here for visible log item 1 ...</tr>
        <tr>... stuff goes here for visible log item 1 ...</tr>
        <tr>... stuff goes here for visible log item 1 ...</tr>
        <tr style="{hidden}">... stuff goes here for COLLAPSED log item 1 ...</tr>
        <tr>... stuff goes here for visible log item 2 ...</tr>
        <tr>... stuff goes here for visible log item 2 ...</tr>
        <tr>... stuff goes here for visible log item 2 ...</tr>
        <tr style="{hidden}">... stuff goes here for COLLAPSED log item 2 ...</tr>
    </table>

And then when the user clicks expand/collapse it's just about toggling visibility for anything to do with "log item N".