admin area: use default layout path, remove turbolinks, load js in async mode, extract menu partial

#341

app/views/layouts/admin.haml

@@ -0,0 +1,36 @@
+!!! 5
+%html{lang: I18n.locale.to_s}
+  %head
+    %meta{charset: "utf-8"}/
+    %meta{content: "width=device-width, initial-scale=1", name: "viewport"}/
+    - if content_for? :head_title
+      = yield :head_title
+    - else
+      %title= t(:admin_head_title)
+    = csrf_meta_tags
+    = stylesheet_link_tag 'admin-manifest', media: 'all'
+    = favicon_link_tag 'favicon.ico'
+  %body{:style => env_style}
+    .navbar.navbar-inverse.navbar-static-top{role: "navigation"}
+      .container
+        .navbar-header
+          %button.navbar-toggle{"data-target" => ".navbar-collapse", "data-toggle" => "collapse", type: "button"}
+            %span.sr-only Toggle navigation
+            %span.icon-bar
+            %span.icon-bar
+            %span.icon-bar
+          = link_to admin_dashboard_path, class: 'navbar-brand' do
+            = ENV['app_name']
+            - if unstable_env.present?
+              .text-center
+                %small{style: 'color: #0074B3;'}= unstable_env
+
+        = render 'menu'
+    .container
+      = render 'shared/flash'
+      = yield
+
+      .footer.text-right
+        Version
+        = CURRENT_COMMIT_HASH
+    = javascript_include_tag 'admin-manifest', async: true