adds feedback button and modal

@drewrwilson @boonrs 

I added a feedback button at the bottom of the page which triggers a simple feedback modal using the existing modal code. Right now the modal just takes what the user inputs and opens the user's default email client to send the email manually (using <code>mailto:denver-ss@codeforamerica.org</code>. Ideally, when the click the 'Send feedback' button, the modal would show a confirmation and the feedback would be mailed to us. 

I also created that new email alias (denver-ss@codeforamerica.org) so people can send us feedback. I can add you as members to the group if you like.