[BACKEND] Scope the events/{id} endpoint to admins only

[bcho892]

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

Because of the changes introduced in #799, we only want users to be able to get the google_forms_link close to the sign up time. However, this change was not introduced for the events/{id} endpoint, as we only want that to exist for admins to see all details about an event.

You will need to move the logic and tests into AdminController.

BEFORE MERGING

• [ ] Integration test written for services
• [ ] Schemas annotated if adding new models
• [ ] Code generation run (hint: yarn workspace server tsoa spec-and-routes)
• [ ] Appropriate mocks created where possible
• [ ] 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