Fixes bug 1067196 - Added columns choice to the Reports tab of signature report.

@peterbe r?

What's cool is that the columns were already supported, all I had to do was enable them in the HTML/JS. Hence the fact that there is no unit test added here: they already exist (see https://github.com/mozilla/socorro/blob/master/webapp-django/crashstats/signature/tests/test_views.py#L144 ). 
Generally this is a bad pattern. The correct pattern is:
```javascript
reportsPanel.on('click', '.controls button', function (e) { ...
```
That means that you have 1 event handler for the whole of the `reportsPanel` object and when that fires, jquery takes care of delegating it to the right object within. 
If you do 
```javascript
reportsPanel.on('click', '.foo', function (e) { ...
reportsPanel.on('click', '.bar', function (e) { ...
```
you still only have 1 event handler. But...
```javascript
$('.foo', reportsPanel).on('click', function (e) { ...
$('.bar', reportsPanel).on('click', function (e) { ...
```
now you have **2** event handlers in the DOM. 

Another benefit, perhaps not clear in this case, with this pattern is that if you do 
```javascript
$('.foo', $ul_element).on('click', function() {...
$('<div>').addClass('foo').appendTo($ul_element);
``` 
then it won't work (because `.foo` doesn't exist yet). However, this *will* work:
```javascript
$ul_element.on('click', '.foo', function() {...
$('<div>').addClass('foo').appendTo($ul_element);
``` 
it will fire the event handler.