Allow adding attendees in simple editor

[ChristophWurst]

Is your feature request related to a problem? Please describe.

As a Nextcloud Calendar user I want to create meetings quickly. One way could be through the simple editor. For attendees I currently have to go to the sidebar editor and switch to the attendee tab.

Describe the solution you'd like

For existing events I'm able to add more attendees right from the simple editor. This feature is missing for (new) events with no attendees.

Describe alternatives you've considered

N/a

Additional context

Follow-up to #3543