Add headings to signin/signup blocks on signin page.

app/assets/stylesheets/sections/login.scss

@@ -40,8 +40,7 @@
     .login-heading h3 {
       font-weight: 300;
       line-height: 1.5;
-      margin: 0;
-      display: none;
+      margin: 0 0 10px 0;
     }
 
     .login-footer {

app/views/devise/shared/_signin_box.html.haml

 .login-box
+  - if signup_enabled?
+    .login-heading
+      %h3 Existing user? Sign in
+  - else
     .login-heading
       %h3 Sign in
   .login-body

app/views/devise/shared/_signup_box.html.haml

 .login-box
+  - if signin_enabled?
     .login-heading
-    %h3 Sign up
+      %h3 New user? Create an account
+  - else
+    .login-heading
+      %h3 Create an account
   .login-body
     = form_for(resource, as: resource_name, url: registration_path(resource_name)) do |f|
       .devise-errors