[BACKEND] Create endpoint for admin to edit event

[choden-dev]

Is your feature request related to a problem? Please describe.

In AdminController we need an endpoint for the fields defined in https://github.com/UoaWDCC/uasc-web/wiki/Firestore-Architecture#event_slot-collection to be edited.

i.e PATCH /admin/events/{id}

BEFORE MERGING

• Storybooks created where possible
• Tested with nock if data fetching
• Tests written for critical interactions
• PR Reviewed (For non-trivial changes)
• Changes tested after rebasing on master or merging in master (hint: git fetch origin master:master, then git rebase master or git merge master)
• All required PR checks passing