[Vue] fix modal notification placement #18377

Description:

Changes:

• Use separate div in modals to display notifications otherwise Vue will erase modal content when initializing NotificationGroup
  component.
• make sure modal notifications have the right group so they are only displayed in the modal.

This should be visible in the TagManager tests.

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

@diosmosis any hint how to reproduce that quickly, so it's easy to confirm if the fix works or not?

@sgiehl in the tag manager, click on manage versions, click import to launch the import modal, then enter invalid json and submit the form ('overwrite current draft with this version'). The notifications will replace the modal content instead of being prepended to it.