[2.0.x] Add support for Taxonomy Terms

Setting this to "Major" as it is a major regression from 1.x and breaks compatibility with submodules like "rabbit_hole_href".

If we'll look into \Drupal\taxonomy\Entity\Term and @ContentEntityType annotation, we'll see that canonical link exists by default:

*   links = {
 *     "canonical" = "/taxonomy/term/{taxonomy_term}",
 *     "delete-form" = "/taxonomy/term/{taxonomy_term}/delete",
 *     "edit-form" = "/taxonomy/term/{taxonomy_term}/edit",
 *     "create" = "/taxonomy/term",
 *   },

We also have proof in the functional tests that "Taxonomy" is available in the supported entity types:

$this->drupalGet('admin/config/content/rabbit-hole');
$this->assertSession()->fieldExists('entity_types[user]');
$this->assertSession()->fieldExists('entity_types[node]');
$this->assertSession()->fieldExists('entity_types[taxonomy_term]');

After that, I noticed this in rabbit_hole_href.module. No surprise it's not working - canonical link for taxonomy terms is not available anymore:

function rabbit_hole_href_entity_type_alter(array &$entity_types) {
  // @todo Determine the entity types which have rabbit_hole_href enabled as
  // rh_action, see Drupal\rabbit_hole\BehaviorInvoker, and go through them
  // instead of only taxonomy_term:
  // behaviourInvorker->getRabbitHoleValuesForEntityType($entity_type, $bundle);
  // NOTE: Currently the uri callback is always set, even if redirect is set to
  // 404 etc.!!
  $entity_types['taxonomy_term']->setUriCallback('rabbit_hole_href_redirect_uri');
  $links = $entity_types['taxonomy_term']->get('links');
  unset($links['canonical']);
  $entity_types['taxonomy_term']->set('links', $links);
}

Maybe the unset is not necessary? For example, in one of the core modules this is not happening:

/**
 * Implements hook_entity_bundle_info_alter().
 */
function forum_entity_bundle_info_alter(&$bundles) {
  // Take over URI construction for taxonomy terms that are forums.
  if ($vid = \Drupal::config('forum.settings')->get('vocabulary')) {
    if (isset($bundles['taxonomy_term'][$vid])) {
      $bundles['taxonomy_term'][$vid]['uri_callback'] = 'forum_uri';
    }
  }
}

Let me know if you think that conditions in Rabbit Hole should be different.

@Matroskeen, thank you! I thought, I'd tested this without rabbit_hole_href enabled! My apologies, I probably forgot to clear all caches, after I uninstalled it...

And thanks for the example from core! I couldn't properly solve this inside 🐛 Using this module in Drupal 10 currently breaks the module Active yesterday, as it is currently broken for the newest Drupal version anyway.