Clarify effect of event type in editor

[PurkkaKoodari]

There are three types of events: "just event", "event + signup", and "just signup". It might be a bit unclear what the types affect in the editor.

Redesign this so it's clearer. This might include always showing (but disabling) tabs when not applicable, or moving/changing the event type field.

Note: The event type is a frontend-only concept. It's identified by the presence of date and registrationStartDate.