Add choice modal to prompt user choice in conversation page

I've added in a modal to make sure that the user wants to archive their conversation when they click the 'Archive conversation' option. This will allow us to perform the appropriate action based on the user's choice.

Here's a screenshot:
![archive modal](https://cloud.githubusercontent.com/assets/5490475/3730667/591c4dd2-16d7-11e4-9dc9-335a54283ced.png)

Veery nice, they all use setAction!  But I think you could simplify these three click event registrations.  Since they all call setAction with one argument, namely their id, I think we could use `this` to get it.  Maybe like:
<pre>
$('#archive, #leave, #delete').click(function(){
    setAction($(this).attr('id'))
});
</pre>
$.attr basically retrieves any attribute stored in the tag, I think