Improve login page for small screens

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/layouts/devise.html.haml

@@ -11,10 +11,10 @@
     .container
       .content
         .row
-          .col-sm-7
+          .col-md-7
             .brand-image
               = brand_image
             .brand_text
               = brand_text
-          .col-sm-5
+          .col-md-5
             = yield