style: if ! changed by unless

- if ! changed by unless as it is more readable
- !object.blank? is equivalent to object.present?, also more readable

app/views/proposal/new.html.haml

@@ -8,7 +8,7 @@
       = render partial: 'encouragement_text'
   .row
     .col-md-8
-      - if !current_user
+      - unless current_user
         %legend
           %span
             =link_to('#signup', role: 'tab', 'aria-controls' => 'home', 'data-toggle' => 'tab') do