Add nuance to same-presenter validation

Problem/Motivation

The validation introduced in ✨ Scheduler Validation: Don't allow more than one session by the same presenter in the same time slot Fixed could create challenges for scheduling admin-created content, such as keynotes.

Proposed resolution

Document current workarounds, such as reassigning the content owner. Potentially offer a configuration option that would leave the current behaviour of visually highlighting conflicts, but allow the session to still be placed (more of a warning instead of enforced validation).