Prevent users from accidentally creating a single activity when they want multiple activities

When creating a new activity with multiple contacts, CiviCRM offers a checkbox to "Create a separate activity for each contact". This is good.

But the "separate activity" use case is common enough, that missing the checkbox can lead to users accidentally creating combined activities. This mistake is bad because it prevents users from being able to make independent edits to those activities for different contacts.

Instead of a checkbox, we should use a set of radio buttons which is empty and required, thus prompting the user to make a decision about whether to create one activity or multiple.