Add additional pre-flight check for asserting a room isn't double booked in TermTime

Description

During the Lent 2024/2025 sync, we observed in TermTime that a room was booked by multiple activities. The student registry do this deliberately so that two cohorts of students can attend the same lectures. Unfortunately, this means the sync service attempts to book the room multiple times in Booker and fails.

The solution to this on the day was for student registry to unschedule one of the cohorts.

Add a pre-flight check to ensure there is no booking conflicts between TermTime activities.

Further details

Comment including a table of the double booked rooms in TermTime. https://gitlab.developers.cam.ac.uk/uis/devops/essm/administration/-/issues/65#note_749627

Acceptance criteria

• New pre-flight check identifies TermTime activities which conflict.
• Process documentation has been updated to include the check.