Clean up role storage on deletion

As far as I can tell, the section information for roles is already cleaned up when a role is deleted. I wrote an automated test for this and tested it manually as well.

I am using:
- Drupal: 8.7.9
- Workbench Access: 1.x-dev
- Palantir.net Drupal Skeleton: https://github.com/palantirnet/drupal-skeleton
- PHPUnit: 6.5.14

- Create two roles, /admin/people/roles/add
- Assign the "Allow all members of this role to be assigned to Workbench Access sections" permission to these roles. (If you assign the permission to authenticated users, it doesn't apply to those roles. :( )
- Create a scheme based on the main navigation, /admin/config/workflow/workbench_access/access_scheme/add, assign it to the Article type that comes with the standard profile
- Assign the two created roles to the top level section
- Confirm the list of roles says "2" and the roles are listed
- Delete one of the roles, /admin/people/roles
- View the list of roles again and confirm it says "1" and the other role is still assigned
- Add another role to the assignment to confirm you can still edit it

The attached patch adds the test for this and updates the interface for role storage. Role IDs in Drupal 8 are strings, not integers.

Let me know if I missed something and there is other behavior we need to test for.

+++ b/tests/src/Functional/DeleteRoleTest.php
@@ -0,0 +1,90 @@
+    $this->assertEquals(['role_a', 'role_b'], $assigned_roles, 'The test roles are not assigned to the section.');
+    ¶
+    /* @var \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager */

nit whitespace here

I think the reference to the role still exists in the section storage, but isn't found because \Drupal\workbench_access\RoleSectionStorage::getRoleSections uses $results = $this->sectionStorage()->loadByProperties(['role_id' => $account->getRoles(), 'access_scheme' => $scheme->id()]); where the call to $account->getRoles() doesn't include the deleted role, because user_user_role_delete does the clean up for the user.

So I think this might still manifest in things where we don't use the getRoleSections method - possibly in views?

+++ b/src/RoleSectionStorage.php
@@ -174,7 +174,7 @@ class RoleSectionStorage implements RoleSectionStorageInterface {
-  protected function saveRoleSections(AccessSchemeInterface $scheme, $role_id, array $settings = []) {
+  public function saveRoleSections(AccessSchemeInterface $scheme, $role_id, array $settings = []) {

is this change needed?

other than that, looks good