Use hash-based flash objects to render complex flash messages

Since flash objects can only be String, Hash or Array (notably excluding SafeBuffers), then this approach is necessary to render complex html in a safe manner.

Each local can be treated as an (unsafe) string, and therefore escaped normally when rendered into the template. The template (and translation strings) can
contain html since they are no longer stored in the flash as a plain string.

Fixes #3215

test/system/confirmation_resend.rb

@@ -0,0 +1,26 @@
+require "application_system_test_case"
+
+class ConfirmationResendSystemTest < ApplicationSystemTestCase
+  def setup
+    @user = build(:user)
+    visit user_new_path
+
+    fill_in "Email", :with => @user.email
+    fill_in "Email Confirmation", :with => @user.email
+    fill_in "Display Name", :with => @user.display_name
+    fill_in "Password", :with => "testtest"
+    fill_in "Confirm Password", :with => "testtest"
+    click_button "Sign Up"
+
+    check "I have read and agree to the above contributor terms"
+    check "I have read and agree to the Terms of Use"
+    click_button "Continue"
+  end
+
+  test "flash message should not contain raw html" do
+    visit user_confirm_resend_path(@user)
+
+    assert page.has_content?("sent a new confirmation")
+    assert_not page.has_content?("<br />")
+  end
+end