Add validation constraints to user.settings

Closes #3436164

core/modules/user/config/schema/user.schema.yml

@@ -3,11 +3,15 @@
 user.settings:
   type: config_object
   label: 'User settings'
+  constraints:
+    FullyValidatable: ~
   mapping:
     anonymous:
+      requiredKey: false
       type: required_label
       label: 'Name'
     verify_mail:
+      requiredKey: false
       type: boolean
       label: 'Require email verification when a visitor creates an account'
     notify:
@@ -15,39 +19,65 @@ user.settings:
       label: 'Notify user'
       mapping:
         cancel_confirm:
+          requiredKey: false
           type: boolean
           label: 'Account cancellation confirmation'
         password_reset:
+          requiredKey: false
           type: boolean
           label: 'Notify user when password reset'
         status_activated:
+          requiredKey: false
           type: boolean
           label: 'Notify user when account is activated'
         status_blocked:
+          requiredKey: false
           type: boolean
           label: 'Account blocked'
         status_canceled:
+          requiredKey: false
           type: boolean
           label: 'Account canceled'
         register_admin_created:
+          requiredKey: false
           type: boolean
           label: 'Welcome (new user created by administrator)'
         register_no_approval_required:
+          requiredKey: false
           type: boolean
           label: 'Welcome (no approval required)'
         register_pending_approval:
+          requiredKey: false
           type: boolean
           label: 'Welcome (awaiting approval)'
     register:
+      requiredKey: false
       type: string
       label: 'Who can register accounts?'
+      # @see \Drupal\user\UserInterface
+      constraints:
+        Choice:
+          choices:
+            - 'visitors'
+            - 'admin_only'
+            - 'visitors_admin_approval'
     cancel_method:
+      requiredKey: false
       type: string
       label: 'When cancelling a user account'
+      nullable: true
+      constraints:
+        NotBlank:
+          allowNull: true
     password_reset_timeout:
+      requiredKey: false
       type: integer
       label: 'Password reset timeout'
+      constraints:
+        Range:
+          min: 0
     password_strength:
+      requiredKey: false
       type: boolean
       label: 'Enable password strength indicator'
 

[Wim Leers]

Missing @sees that explain these choices.

[Wim Leers]

Missing @see that explains this exceptional case.