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/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