Use a post link to logout

This avoids needing to access the session id, which is currently
only working with the memcache store.

The fallback page is preserved for anyone who wants to logout without
using javascript.

Refs #2488

test/controllers/users_controller_test.rb

@@ -344,27 +344,13 @@ class UsersControllerTest < ActionController::TestCase
   end
 
   def test_logout_without_referer
-    get :logout
-    assert_response :success
-    assert_template :logout
-    assert_select "input[name=referer][value=?]", ""
-
-    session_id = assert_select("input[name=session]").first["value"]
-
-    get :logout, :params => { :session => session_id }
+    post :logout
     assert_response :redirect
     assert_redirected_to root_path
   end
 
   def test_logout_with_referer
-    get :logout, :params => { :referer => "/test" }
-    assert_response :success
-    assert_template :logout
-    assert_select "input[name=referer][value=?]", "/test"
-
-    session_id = assert_select("input[name=session]").first["value"]
-
-    get :logout, :params => { :session => session_id, :referer => "/test" }
+    post :logout, :params => { :referer => "/test" }
     assert_response :redirect
     assert_redirected_to "/test"
   end
@@ -374,16 +360,7 @@ class UsersControllerTest < ActionController::TestCase
 
     session[:token] = token.token
 
-    get :logout
-    assert_response :success
-    assert_template :logout
-    assert_select "input[name=referer][value=?]", ""
-    assert_equal token.token, session[:token]
-    assert_not_nil UserToken.where(:id => token.id).first
-
-    session_id = assert_select("input[name=session]").first["value"]
-
-    get :logout, :params => { :session => session_id }
+    post :logout
     assert_response :redirect
     assert_redirected_to root_path
     assert_nil session[:token]

test/system/user_logout_test.rb

@@ -0,0 +1,22 @@
+require "application_system_test_case"
+
+class UserLogoutTest < ApplicationSystemTestCase
+  test "Sign out via link" do
+    user = create(:user)
+    sign_in_as(user)
+
+    click_on user.display_name
+    click_on "Log Out"
+    assert page.has_content? "Log In"
+  end
+
+  test "Sign out via fallback page" do
+    sign_in_as(create(:user))
+
+    visit logout_path
+    assert page.has_content? "Logout from OpenStreetMap"
+
+    click_button "Logout"
+    assert page.has_content? "Log In"
+  end
+end