Improve login screen when only OmniAuth providers are enabled

Hide the text "No authentication methods configured." when Signin and LDAP are disabled but OmniAuth is enabled.
Hide the link to "Did not receive confirmation email?" when Signup is disabled.
Only show the horizontal rule between the login form and omniauth buttons if both are enabled

app/views/devise/sessions/new.html.haml

@@ -2,6 +2,7 @@
   .login-heading
     %h3 Sign in
   .login-body
+    - no_auth = 0
     - if ldap_enabled? && gitlab_config.signin_enabled
       %ul.nav.nav-tabs
         %li.active
@@ -19,23 +20,29 @@
     - elsif gitlab_config.signin_enabled
       = render partial: 'devise/sessions/new_base'
     - else
+      - no_auth += 1
+
+    - if Gitlab.config.omniauth.enabled && devise_mapping.omniauthable?
+      - if no_auth == 0
+        %hr
+      = render partial: 'devise/sessions/oauth_providers'
+    - else
+      - no_auth += 1
+
+    - if no_auth == 2
       %div
         No authentication methods configured.
 
-    = render 'devise/sessions/oauth_providers' if Gitlab.config.omniauth.enabled && devise_mapping.omniauthable?
-
   .login-footer
     - if gitlab_config.signup_enabled
       %p
         %span.light
           Don't have an account?
         %strong
           = link_to "Sign up", new_registration_path(resource_name)
-
-    %p
-      %span.light Did not receive confirmation email?
-      = link_to "Send again", new_confirmation_path(resource_name)
-
+      %p
+        %span.light Did not receive confirmation email?
+        = link_to "Send again", new_confirmation_path(resource_name)
 
     - if extra_config.has_key?('sign_in_text')
       %hr