Follow-up to Update honeypot module to use form state interface

This patch is a follow-up to #2322247: Update honeypot module to use form state interface and finishes the clean-up related to $form_state being a classed object in Drupal 8.

At the same time, it also fixes two minor issues that wouldn't probably need their own dedicated issue:

• Rename honeypot.menu_links.yml to honeypot.links.menu.yml (see https://www.drupal.org/node/2302069)
• Convert static permissions from hook_permissions() to honeypot.permissions.yml (see https://www.drupal.org/node/2311427)