Render `gon` data in the page `body`, not `head`

- Turbolinks caches the `head`, so `gon` updates don't show up unless
  the user navigates to page directly (by URL) or performs a refresh.
- The solution is to render `gon` in the body instead.
- Also update the syntax to the new Rails 4 (according to the gon
  README) syntax.

app/views/layouts/_head.html.haml

@@ -35,8 +35,6 @@
 
   = csrf_meta_tags
 
-  = include_gon
-
   - unless browser.safari?
     %meta{name: 'referrer', content: 'origin-when-cross-origin'}
   %meta{name: 'viewport', content: 'width=device-width, initial-scale=1, maximum-scale=1'}

app/views/layouts/application.html.haml

@@ -2,6 +2,8 @@
 %html{ lang: "en"}
   = render "layouts/head"
   %body{class: "#{user_application_theme}", 'data-page' => body_data_page}
+    = Gon::Base.render_data
+
     -# Ideally this would be inside the head, but turbolinks only evaluates page-specific JS in the body.
     = yield :scripts_body_top
 

app/views/layouts/devise.html.haml

@@ -2,6 +2,7 @@
 %html{ lang: "en"}
   = render "layouts/head"
   %body.ui_charcoal.login-page.application.navless
+    = Gon::Base.render_data
     = render "layouts/header/empty"
     = render "layouts/broadcast"
     .container.navless-container

app/views/layouts/devise_empty.html.haml

@@ -2,6 +2,7 @@
 %html{ lang: "en"}
   = render "layouts/head"
   %body.ui_charcoal.login-page.application.navless
+    = Gon::Base.render_data
     = render "layouts/header/empty"
     = render "layouts/broadcast"
     .container.navless-container

app/views/layouts/errors.html.haml

@@ -2,6 +2,7 @@
 %html{ lang: "en"}
   = render "layouts/head"
   %body{class: "#{user_application_theme} application navless"}
+    = Gon::Base.render_data
     = render "layouts/header/empty"
     .container.navless-container
       = render "layouts/flash"