Hide contact blurb on mobile until col refactor

Mobile layout handling should be more pragmatic, so that should be refactored before we re-tweak the entire mobile layout to make room for this notice.

Author: peterkos

src/preregister.js

@@ -34,7 +34,7 @@ class PreRegister extends React.Component {
 
                         <MailchimpInput/>
 
-                        <div id="contact">
+                        <div id="contact" className="mobile-hide">
                             <p>interested in sponsoring?</p>
                             <p>email <a href="mailto:sponsorship@coderit.org">sponsorship@coderit.org</a></p>
                         </div>