Add gitlab logo on login page

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

app/views/layouts/devise.html.haml

@@ -2,6 +2,11 @@
 %html{ lang: "en"}
   = render "layouts/head"
   %body.ui_mars.login-page.application
+    %header.navbar.navbar-fixed-top.navbar-gitlab
+      .navbar-inner
+        .container
+          %h4.center
+            = image_tag 'logo-white.png', width: 32, height: 32
     = render "layouts/broadcast"
     .container.navless-container
       .content