Improve role/capability handling in usermanager #19388

Description:

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

Review

• [ ] Functional review done
• [ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done
• [ ] Wording review done
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed

This one should be ready for review now. I've already tried to explain some of the changes in some comments above. Hope that makes it easier to understand everything.