FIX: do not show "Send Activation Email" button if `must_approve_users` setting is enabled

https://meta.discourse.org/t/error-when-pressing-send-activitation-email-when-must-approve-users-setting-is-enabled/65408

app/serializers/admin_user_serializer.rb

@@ -8,8 +8,7 @@ class AdminUserSerializer < AdminUserListSerializer
              :can_activate,
              :can_deactivate,
              :ip_address,
-             :registration_ip_address,
-             :can_send_activation_email
+             :registration_ip_address
 
   has_one :single_sign_on_record, serializer: SingleSignOnRecordSerializer, embed: :objects
 

lib/guardian.rb

@@ -126,9 +126,12 @@ class Guardian
   end
   alias :can_move_posts? :can_moderate?
   alias :can_see_flags? :can_moderate?
-  alias :can_send_activation_email? :can_moderate?
   alias :can_close? :can_moderate?
 
+  def can_send_activation_email?(user)
+    user && is_staff? && !SiteSetting.must_approve_users?
+  end
+
   def can_grant_badges?(_user)
     SiteSetting.enable_badges && is_staff?
   end