Project or repo admin can't see list of event types to select when adding or editing an event handler

To reproduce add a repository level event handler like send mail. Notice that the list of events to select from is never provided.

This is because the events endpoint that the list of events is retrieved from is locked down to admins only and returns 403.

We should open it up to project and repo admins which will return those events that are only available to repo and project admins.