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