Ability for admins to undo marking a response as a clarification request

[RichardTaylor]

We'd also want to undo the record of the thread being consequently put in the status awaiting_clarification

The aim would be that undoing this would undo the impact on the calculation of the deadline.

We've managed to run the service for a long time before realising we didn't have this feature so it's probably not commonly needed and not a priority.