FIX: Cleanup authentication_data cookie after login (#11834)

This cookie is only used during login. Having it persist after that can
cause some unusual behavior, especially for sites with short session
lengths.

We were already deleting the cookie following a new signup, but not for
existing users.

This commit moves the cookie deletion logic out of the erb template, and
adds logic and tests to ensure it is always deleted consistently.

Co-authored-by: Jarek Radosz <jradosz@gmail.com>

app/helpers/application_helper.rb

@@ -560,4 +560,16 @@ module ApplicationHelper
       end
     end
   end
+
+  def authentication_data
+    return @authentication_data if defined?(@authentication_data)
+
+    @authentication_data = begin
+      value = cookies[:authentication_data]
+      if value
+        cookies.delete(:authentication_data, path: Discourse.base_path("/"))
+      end
+      current_user ? nil : value
+    end
+  end
 end