Turn recurring sidebar event into regular event after dropping on grid

[tyler-dane]

Currently, the recurrence values of a sidebar event are maintained after dropping it on the grid.
For simplicity and clarity, handle this case as a regular event by removing the recurrence properties.

This will let the user understand that the event is now a regular event and its original recurrence from the sidebar no longer applies now that it's on the main grid.

[tyler-dane]

@that-one-arab I assigned this to you because I've still got my hands full with the recurrence backend changes, and this can be done independent of that.

[that-one-arab]

@tyler-dane Can you clarify this one please?

• Are we to remove the recurrence field from the event object?
  
• Or are we to remove the recurrence fields in the event form?
  

[tyler-dane]

The former - we need to strip the irrelevant recurrence properties after dropping, so that they don't make it into the DB as part of the update request. This'll help us avoid future scenarios, where an event seems like a recurrence but actually isn't