DND day task to agenda

[tyler-dane]

Priority

High (would significantly improve my experience)

Feature Description

When dragging a task, the user should be able to either reorder it in the task list OR drop it onto the agenda surface and turn it into an event.

When it is dropped on the agenda, the task should be deleted and the event should be created.

The agenda component should handle optimistic rendering, so that the event appears immediately.

Errors should be handled if there was any issue with the event creation.

Use Case

This'll help users who want to timeblock by moving tasks onto the calendar.

Additional Context

• ensure this is well tested and the code is DRY
• Out of scope: undoing a drop from a task to an event. We will do this in a separate PR, but keep that functionality in mind.