Create a custom validator for newsletters

Now that we're no longer reusing the List metadata field in the
Resource, we have to adapt the validators.

validators.go

@@ -582,6 +582,26 @@ func (v *validationContext) validListResource() ResourceValidatorFunc {
 	}
 }
 
+func (v *validationContext) validNewsletterResource() ResourceValidatorFunc {
+	listValidator := v.validHostedMailingList()
+
+	return func(ctx context.Context, r *Resource, user *User) error {
+		if err := v.validateShardedResource(ctx, r, user); err != nil {
+			return err
+		}
+		if r.Newsletter == nil {
+			return errors.New("resource has no newsletter metadata")
+		}
+		if err := listValidator(ctx, r.Name); err != nil {
+			return err
+		}
+		if len(r.Newsletter.Admins) < 1 {
+			return errors.New("can't create a newsletter without admins")
+		}
+		return nil
+	}
+}
+
 func findMatchingDAVAccount(user *User, r *Resource) *Resource {
 	for _, dav := range user.GetResourcesByType(ResourceTypeDAV) {
 		if isSubdir(dav.DAV.Homedir, r.Website.DocumentRoot) {
@@ -762,7 +782,7 @@ func newResourceValidator(v *validationContext) *resourceValidator {
 		rvs: map[string]ResourceValidatorFunc{
 			ResourceTypeEmail:       v.validEmailResource(),
 			ResourceTypeMailingList: v.validListResource(),
-			ResourceTypeNewsletter:  v.validListResource(),
+			ResourceTypeNewsletter:  v.validNewsletterResource(),
 			ResourceTypeDomain:      v.validDomainResource(),
 			ResourceTypeWebsite:     v.validWebsiteResource(),
 			ResourceTypeDAV:         v.validDAVResource(),