Add archive modal to prompt user choice

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)