While trying to fix #18750 a couple of other issue within the UserManager popped up. Guess some of them might have regressed during the vue migration.
Note: This PR also adds a proper handling of bulk requests to AjaxHandler. This is e.g. important when setting a role/capability for multiple users at once. The requests are sent as bulk. Currently it would not show any error if some of the requests failed.
fixes #18750